partly or unimplemented features - now implemented

src/Policy/StellaOps.Policy.Engine/Endpoints/DeltaIfPresentEndpoints.cs

@@ -0,0 +1,529 @@
+// <copyright file="DeltaIfPresentEndpoints.cs" company="StellaOps">
+// SPDX-License-Identifier: BUSL-1.1
+// Sprint: SPRINT_20260208_043_Policy_delta_if_present_calculations_for_missing_signals (TSF-004)
+// </copyright>
+
+using System.Text.Json.Serialization;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Routing;
+using Microsoft.Extensions.Logging;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring;
+
+namespace StellaOps.Policy.Engine.Endpoints;
+
+/// <summary>
+/// API endpoints for delta-if-present calculations (TSF-004).
+/// Shows hypothetical score changes when missing signals are filled with assumed values.
+/// </summary>
+public static class DeltaIfPresentEndpoints
+{
+    /// <summary>
+    /// Maps delta-if-present endpoints.
+    /// </summary>
+    public static IEndpointRouteBuilder MapDeltaIfPresentEndpoints(this IEndpointRouteBuilder endpoints)
+    {
+        var group = endpoints.MapGroup("/api/v1/policy/delta-if-present")
+            .WithTags("Delta If Present")
+            .WithOpenApi();
+
+        // Calculate single signal delta
+        group.MapPost("/signal", CalculateSingleSignalDeltaAsync)
+            .WithName("CalculateSingleSignalDelta")
+            .WithSummary("Calculate hypothetical score change for a single signal")
+            .WithDescription("Shows what the trust score would be if a specific missing signal had a particular value")
+            .Produces<SingleSignalDeltaResponse>(StatusCodes.Status200OK)
+            .Produces<ProblemDetails>(StatusCodes.Status400BadRequest)
+            .RequireAuthorization("PolicyViewer");
+
+        // Calculate full gap analysis
+        group.MapPost("/analysis", CalculateFullAnalysisAsync)
+            .WithName("CalculateFullGapAnalysis")
+            .WithSummary("Calculate full gap analysis for all missing signals")
+            .WithDescription("Analyzes all signal gaps with best/worst/prior case scenarios and prioritization by impact")
+            .Produces<FullAnalysisResponse>(StatusCodes.Status200OK)
+            .Produces<ProblemDetails>(StatusCodes.Status400BadRequest)
+            .RequireAuthorization("PolicyViewer");
+
+        // Calculate score bounds
+        group.MapPost("/bounds", CalculateScoreBoundsAsync)
+            .WithName("CalculateScoreBounds")
+            .WithSummary("Calculate minimum and maximum possible scores")
+            .WithDescription("Computes the range of possible trust scores given current gaps")
+            .Produces<ScoreBoundsResponse>(StatusCodes.Status200OK)
+            .Produces<ProblemDetails>(StatusCodes.Status400BadRequest)
+            .RequireAuthorization("PolicyViewer");
+
+        return endpoints;
+    }
+
+    private static IResult CalculateSingleSignalDeltaAsync(
+        [FromBody] SingleSignalDeltaRequest request,
+        IDeltaIfPresentCalculator calculator,
+        ILogger<DeltaIfPresentEndpoints> logger)
+    {
+        if (request.Snapshot is null)
+        {
+            return Results.BadRequest(new ProblemDetails
+            {
+                Title = "Invalid request",
+                Detail = "Snapshot is required"
+            });
+        }
+
+        if (string.IsNullOrEmpty(request.SignalName))
+        {
+            return Results.BadRequest(new ProblemDetails
+            {
+                Title = "Invalid request",
+                Detail = "SignalName is required"
+            });
+        }
+
+        logger.LogDebug(
+            "Calculating single signal delta for {Signal} with assumed value {Value}",
+            request.SignalName,
+            request.AssumedValue);
+
+        var result = calculator.CalculateSingleSignalDelta(
+            request.Snapshot,
+            request.SignalName,
+            request.AssumedValue,
+            request.CustomWeights);
+
+        return Results.Ok(new SingleSignalDeltaResponse
+        {
+            Signal = result.Signal,
+            CurrentScore = result.CurrentScore,
+            HypotheticalScore = result.HypotheticalScore,
+            ScoreDelta = result.Delta,
+            AssumedValue = result.AssumedValue,
+            SignalWeight = result.SignalWeight,
+            CurrentEntropy = result.CurrentEntropy,
+            HypotheticalEntropy = result.HypotheticalEntropy,
+            EntropyDelta = result.EntropyDelta
+        });
+    }
+
+    private static IResult CalculateFullAnalysisAsync(
+        [FromBody] FullAnalysisRequest request,
+        IDeltaIfPresentCalculator calculator,
+        ILogger<DeltaIfPresentEndpoints> logger)
+    {
+        if (request.Snapshot is null)
+        {
+            return Results.BadRequest(new ProblemDetails
+            {
+                Title = "Invalid request",
+                Detail = "Snapshot is required"
+            });
+        }
+
+        logger.LogDebug(
+            "Calculating full gap analysis for CVE {Cve}, PURL {Purl}",
+            request.Snapshot.Cve,
+            request.Snapshot.Purl);
+
+        var analysis = calculator.CalculateFullAnalysis(request.Snapshot, request.CustomWeights);
+
+        var gaps = analysis.GapAnalysis.Select(g => new GapAnalysisItemResponse
+        {
+            Signal = g.BestCase.Signal,
+            GapReason = g.GapReason.ToString(),
+            BestCase = MapDeltaResult(g.BestCase),
+            WorstCase = MapDeltaResult(g.WorstCase),
+            PriorCase = MapDeltaResult(g.PriorCase),
+            MaxImpact = g.MaxImpact
+        }).ToList();
+
+        return Results.Ok(new FullAnalysisResponse
+        {
+            Cve = request.Snapshot.Cve,
+            Purl = request.Snapshot.Purl,
+            CurrentScore = analysis.CurrentScore,
+            CurrentEntropy = analysis.CurrentEntropy,
+            GapAnalysis = gaps,
+            PrioritizedGaps = analysis.PrioritizedGaps.ToList(),
+            ComputedAt = analysis.ComputedAt
+        });
+    }
+
+    private static IResult CalculateScoreBoundsAsync(
+        [FromBody] ScoreBoundsRequest request,
+        IDeltaIfPresentCalculator calculator,
+        ILogger<DeltaIfPresentEndpoints> logger)
+    {
+        if (request.Snapshot is null)
+        {
+            return Results.BadRequest(new ProblemDetails
+            {
+                Title = "Invalid request",
+                Detail = "Snapshot is required"
+            });
+        }
+
+        logger.LogDebug(
+            "Calculating score bounds for CVE {Cve}, PURL {Purl}",
+            request.Snapshot.Cve,
+            request.Snapshot.Purl);
+
+        var bounds = calculator.CalculateScoreBounds(request.Snapshot, request.CustomWeights);
+
+        return Results.Ok(new ScoreBoundsResponse
+        {
+            Cve = request.Snapshot.Cve,
+            Purl = request.Snapshot.Purl,
+            CurrentScore = bounds.CurrentScore,
+            CurrentEntropy = bounds.CurrentEntropy,
+            MinimumScore = bounds.MinimumScore,
+            MaximumScore = bounds.MaximumScore,
+            Range = bounds.Range,
+            GapCount = bounds.GapCount,
+            MissingWeightPercentage = bounds.MissingWeightPercentage,
+            ComputedAt = bounds.ComputedAt
+        });
+    }
+
+    private static DeltaResultResponse MapDeltaResult(DeltaIfPresentResult result)
+    {
+        return new DeltaResultResponse
+        {
+            AssumedValue = result.AssumedValue,
+            HypotheticalScore = result.HypotheticalScore,
+            ScoreDelta = result.Delta,
+            HypotheticalEntropy = result.HypotheticalEntropy,
+            EntropyDelta = result.EntropyDelta
+        };
+    }
+}
+
+#region Request DTOs
+
+/// <summary>
+/// Request to calculate delta for a single signal.
+/// </summary>
+public sealed record SingleSignalDeltaRequest
+{
+    /// <summary>
+    /// The current signal snapshot.
+    /// </summary>
+    [JsonPropertyName("snapshot")]
+    public required SignalSnapshot Snapshot { get; init; }
+
+    /// <summary>
+    /// Name of the signal to simulate (VEX, EPSS, Reachability, Runtime, Backport, SBOMLineage).
+    /// </summary>
+    [JsonPropertyName("signal_name")]
+    public required string SignalName { get; init; }
+
+    /// <summary>
+    /// The assumed value for the signal (0.0 to 1.0 where 0 = lowest risk, 1 = highest risk).
+    /// </summary>
+    [JsonPropertyName("assumed_value")]
+    public double AssumedValue { get; init; }
+
+    /// <summary>
+    /// Optional custom signal weights. If not provided, defaults are used.
+    /// </summary>
+    [JsonPropertyName("custom_weights")]
+    public SignalWeights? CustomWeights { get; init; }
+}
+
+/// <summary>
+/// Request to calculate full gap analysis.
+/// </summary>
+public sealed record FullAnalysisRequest
+{
+    /// <summary>
+    /// The current signal snapshot.
+    /// </summary>
+    [JsonPropertyName("snapshot")]
+    public required SignalSnapshot Snapshot { get; init; }
+
+    /// <summary>
+    /// Optional custom signal weights. If not provided, defaults are used.
+    /// </summary>
+    [JsonPropertyName("custom_weights")]
+    public SignalWeights? CustomWeights { get; init; }
+}
+
+/// <summary>
+/// Request to calculate score bounds.
+/// </summary>
+public sealed record ScoreBoundsRequest
+{
+    /// <summary>
+    /// The current signal snapshot.
+    /// </summary>
+    [JsonPropertyName("snapshot")]
+    public required SignalSnapshot Snapshot { get; init; }
+
+    /// <summary>
+    /// Optional custom signal weights. If not provided, defaults are used.
+    /// </summary>
+    [JsonPropertyName("custom_weights")]
+    public SignalWeights? CustomWeights { get; init; }
+}
+
+#endregion
+
+#region Response DTOs
+
+/// <summary>
+/// Response for single signal delta calculation.
+/// </summary>
+public sealed record SingleSignalDeltaResponse
+{
+    /// <summary>
+    /// Name of the signal analyzed.
+    /// </summary>
+    [JsonPropertyName("signal")]
+    public required string Signal { get; init; }
+
+    /// <summary>
+    /// Current trust score.
+    /// </summary>
+    [JsonPropertyName("current_score")]
+    public double CurrentScore { get; init; }
+
+    /// <summary>
+    /// Hypothetical score if the signal had the assumed value.
+    /// </summary>
+    [JsonPropertyName("hypothetical_score")]
+    public double HypotheticalScore { get; init; }
+
+    /// <summary>
+    /// Change in score (hypothetical - current).
+    /// </summary>
+    [JsonPropertyName("score_delta")]
+    public double ScoreDelta { get; init; }
+
+    /// <summary>
+    /// The assumed value used for simulation.
+    /// </summary>
+    [JsonPropertyName("assumed_value")]
+    public double AssumedValue { get; init; }
+
+    /// <summary>
+    /// Weight of the signal in scoring.
+    /// </summary>
+    [JsonPropertyName("signal_weight")]
+    public double SignalWeight { get; init; }
+
+    /// <summary>
+    /// Current entropy (uncertainty).
+    /// </summary>
+    [JsonPropertyName("current_entropy")]
+    public double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// Hypothetical entropy after adding the signal.
+    /// </summary>
+    [JsonPropertyName("hypothetical_entropy")]
+    public double HypotheticalEntropy { get; init; }
+
+    /// <summary>
+    /// Change in entropy (negative = less uncertainty).
+    /// </summary>
+    [JsonPropertyName("entropy_delta")]
+    public double EntropyDelta { get; init; }
+}
+
+/// <summary>
+/// Response for full gap analysis.
+/// </summary>
+public sealed record FullAnalysisResponse
+{
+    /// <summary>
+    /// CVE identifier.
+    /// </summary>
+    [JsonPropertyName("cve")]
+    public required string Cve { get; init; }
+
+    /// <summary>
+    /// Package URL.
+    /// </summary>
+    [JsonPropertyName("purl")]
+    public required string Purl { get; init; }
+
+    /// <summary>
+    /// Current trust score.
+    /// </summary>
+    [JsonPropertyName("current_score")]
+    public double CurrentScore { get; init; }
+
+    /// <summary>
+    /// Current entropy (uncertainty).
+    /// </summary>
+    [JsonPropertyName("current_entropy")]
+    public double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// Analysis of each signal gap with best/worst/prior cases.
+    /// </summary>
+    [JsonPropertyName("gap_analysis")]
+    public required IReadOnlyList<GapAnalysisItemResponse> GapAnalysis { get; init; }
+
+    /// <summary>
+    /// Signals prioritized by maximum impact (highest first).
+    /// </summary>
+    [JsonPropertyName("prioritized_gaps")]
+    public required IReadOnlyList<string> PrioritizedGaps { get; init; }
+
+    /// <summary>
+    /// Timestamp when analysis was computed.
+    /// </summary>
+    [JsonPropertyName("computed_at")]
+    public DateTimeOffset ComputedAt { get; init; }
+}
+
+/// <summary>
+/// Individual gap analysis result.
+/// </summary>
+public sealed record GapAnalysisItemResponse
+{
+    /// <summary>
+    /// Name of the signal.
+    /// </summary>
+    [JsonPropertyName("signal")]
+    public required string Signal { get; init; }
+
+    /// <summary>
+    /// Reason for the gap.
+    /// </summary>
+    [JsonPropertyName("gap_reason")]
+    public required string GapReason { get; init; }
+
+    /// <summary>
+    /// Best case scenario (lowest risk assumption).
+    /// </summary>
+    [JsonPropertyName("best_case")]
+    public required DeltaResultResponse BestCase { get; init; }
+
+    /// <summary>
+    /// Worst case scenario (highest risk assumption).
+    /// </summary>
+    [JsonPropertyName("worst_case")]
+    public required DeltaResultResponse WorstCase { get; init; }
+
+    /// <summary>
+    /// Prior case scenario (prior probability assumption).
+    /// </summary>
+    [JsonPropertyName("prior_case")]
+    public required DeltaResultResponse PriorCase { get; init; }
+
+    /// <summary>
+    /// Maximum possible score impact (worst - best).
+    /// </summary>
+    [JsonPropertyName("max_impact")]
+    public double MaxImpact { get; init; }
+}
+
+/// <summary>
+/// Delta result for a specific scenario.
+/// </summary>
+public sealed record DeltaResultResponse
+{
+    /// <summary>
+    /// Assumed value for the signal.
+    /// </summary>
+    [JsonPropertyName("assumed_value")]
+    public double AssumedValue { get; init; }
+
+    /// <summary>
+    /// Hypothetical score with assumed value.
+    /// </summary>
+    [JsonPropertyName("hypothetical_score")]
+    public double HypotheticalScore { get; init; }
+
+    /// <summary>
+    /// Change in score.
+    /// </summary>
+    [JsonPropertyName("score_delta")]
+    public double ScoreDelta { get; init; }
+
+    /// <summary>
+    /// Hypothetical entropy with assumed value.
+    /// </summary>
+    [JsonPropertyName("hypothetical_entropy")]
+    public double HypotheticalEntropy { get; init; }
+
+    /// <summary>
+    /// Change in entropy.
+    /// </summary>
+    [JsonPropertyName("entropy_delta")]
+    public double EntropyDelta { get; init; }
+}
+
+/// <summary>
+/// Response for score bounds calculation.
+/// </summary>
+public sealed record ScoreBoundsResponse
+{
+    /// <summary>
+    /// CVE identifier.
+    /// </summary>
+    [JsonPropertyName("cve")]
+    public required string Cve { get; init; }
+
+    /// <summary>
+    /// Package URL.
+    /// </summary>
+    [JsonPropertyName("purl")]
+    public required string Purl { get; init; }
+
+    /// <summary>
+    /// Current trust score.
+    /// </summary>
+    [JsonPropertyName("current_score")]
+    public double CurrentScore { get; init; }
+
+    /// <summary>
+    /// Current entropy (uncertainty).
+    /// </summary>
+    [JsonPropertyName("current_entropy")]
+    public double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// Minimum possible score (all gaps at best case).
+    /// </summary>
+    [JsonPropertyName("minimum_score")]
+    public double MinimumScore { get; init; }
+
+    /// <summary>
+    /// Maximum possible score (all gaps at worst case).
+    /// </summary>
+    [JsonPropertyName("maximum_score")]
+    public double MaximumScore { get; init; }
+
+    /// <summary>
+    /// Range of possible scores.
+    /// </summary>
+    [JsonPropertyName("range")]
+    public double Range { get; init; }
+
+    /// <summary>
+    /// Number of signal gaps.
+    /// </summary>
+    [JsonPropertyName("gap_count")]
+    public int GapCount { get; init; }
+
+    /// <summary>
+    /// Percentage of total weight that is missing.
+    /// </summary>
+    [JsonPropertyName("missing_weight_percentage")]
+    public double MissingWeightPercentage { get; init; }
+
+    /// <summary>
+    /// Timestamp when bounds were computed.
+    /// </summary>
+    [JsonPropertyName("computed_at")]
+    public DateTimeOffset ComputedAt { get; init; }
+}
+
+#endregion
+
+// Logger interface for typed logging
+internal sealed class DeltaIfPresentEndpoints { }

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/CombinedImpactCalculator.cs

@@ -0,0 +1,114 @@
+using StellaOps.Policy.Determinization.Models;
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Combined score result that integrates impact and uncertainty scores.
+/// </summary>
+public sealed record CombinedImpactScore
+{
+    /// <summary>Impact score from multi-factor calculation.</summary>
+    [JsonPropertyName("impact")]
+    public required ImpactScore Impact { get; init; }
+
+    /// <summary>Uncertainty score from entropy calculation.</summary>
+    [JsonPropertyName("uncertainty")]
+    public required UncertaintyScore Uncertainty { get; init; }
+
+    /// <summary>
+    /// Effective priority score combining impact and uncertainty.
+    /// Higher uncertainty reduces the effective priority.
+    /// Formula: impact * (1 - uncertainty_entropy * uncertainty_penalty_factor)
+    /// </summary>
+    [JsonPropertyName("effective_priority")]
+    public required double EffectivePriority { get; init; }
+
+    /// <summary>Basis points representation of effective priority (0-10000).</summary>
+    [JsonPropertyName("effective_priority_basis_points")]
+    public required int EffectivePriorityBasisPoints { get; init; }
+
+    /// <summary>When this combined score was calculated (UTC).</summary>
+    [JsonPropertyName("calculated_at")]
+    public required DateTimeOffset CalculatedAt { get; init; }
+}
+
+/// <summary>
+/// Interface for combined impact-uncertainty score calculation.
+/// </summary>
+public interface ICombinedImpactCalculator
+{
+    /// <summary>
+    /// Calculates combined impact-uncertainty score for prioritization.
+    /// </summary>
+    /// <param name="impactContext">Impact context with environment, data sensitivity, etc.</param>
+    /// <param name="signalSnapshot">Signal snapshot for uncertainty calculation.</param>
+    /// <param name="uncertaintyPenaltyFactor">How much uncertainty reduces priority (default 0.5).</param>
+    /// <returns>Combined score with impact, uncertainty, and effective priority.</returns>
+    CombinedImpactScore Calculate(
+        ImpactContext impactContext,
+        SignalSnapshot signalSnapshot,
+        double uncertaintyPenaltyFactor = 0.5);
+}
+
+/// <summary>
+/// Calculates combined impact-uncertainty scores for unknown triage.
+/// Integrates ImpactScoreCalculator with UncertaintyScoreCalculator for
+/// a unified prioritization signal.
+/// </summary>
+public sealed class CombinedImpactCalculator : ICombinedImpactCalculator
+{
+    private readonly IImpactScoreCalculator _impactCalculator;
+    private readonly IUncertaintyScoreCalculator _uncertaintyCalculator;
+    private readonly ILogger<CombinedImpactCalculator> _logger;
+    private readonly TimeProvider _timeProvider;
+
+    public CombinedImpactCalculator(
+        IImpactScoreCalculator impactCalculator,
+        IUncertaintyScoreCalculator uncertaintyCalculator,
+        ILogger<CombinedImpactCalculator> logger,
+        TimeProvider? timeProvider = null)
+    {
+        _impactCalculator = impactCalculator;
+        _uncertaintyCalculator = uncertaintyCalculator;
+        _logger = logger;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+    }
+
+    /// <inheritdoc />
+    public CombinedImpactScore Calculate(
+        ImpactContext impactContext,
+        SignalSnapshot signalSnapshot,
+        double uncertaintyPenaltyFactor = 0.5)
+    {
+        ArgumentNullException.ThrowIfNull(impactContext);
+        ArgumentNullException.ThrowIfNull(signalSnapshot);
+
+        // Calculate individual scores
+        var impact = _impactCalculator.Calculate(impactContext);
+        var uncertainty = _uncertaintyCalculator.Calculate(signalSnapshot);
+
+        // Effective priority = impact * (1 - uncertainty * penalty)
+        // When entropy is high, priority is reduced
+        var penaltyFactor = Math.Clamp(uncertaintyPenaltyFactor, 0.0, 1.0);
+        var effectivePriority = impact.Score * (1.0 - uncertainty.Entropy * penaltyFactor);
+        effectivePriority = Math.Clamp(effectivePriority, 0.0, 1.0);
+        var effectivePriorityBasisPoints = (int)Math.Round(effectivePriority * 10000);
+
+        _logger.LogDebug(
+            "Calculated combined score: impact={Impact:F4}, uncertainty={Uncertainty:F4}, effective={Effective:F4} (penalty_factor={PenaltyFactor:F2})",
+            impact.Score,
+            uncertainty.Entropy,
+            effectivePriority,
+            penaltyFactor);
+
+        return new CombinedImpactScore
+        {
+            Impact = impact,
+            Uncertainty = uncertainty,
+            EffectivePriority = effectivePriority,
+            EffectivePriorityBasisPoints = effectivePriorityBasisPoints,
+            CalculatedAt = _timeProvider.GetUtcNow()
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/DeltaIfPresentCalculator.cs

@@ -0,0 +1,346 @@
+// <copyright file="DeltaIfPresentCalculator.cs" company="StellaOps">
+// Copyright (c) StellaOps. Licensed under the BUSL-1.1.
+// </copyright>
+
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Calculates hypothetical score changes if missing signals were present.
+/// Implements TSF-004: Delta-If-Present calculations for policy decision support.
+/// </summary>
+public sealed class DeltaIfPresentCalculator : IDeltaIfPresentCalculator
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization");
+    private static readonly Counter<long> DeltaCalculationsCounter = Meter.CreateCounter<long>(
+        "stellaops_determinization_delta_if_present_calculations_total",
+        description: "Total delta-if-present calculations performed");
+
+    private readonly ILogger<DeltaIfPresentCalculator> _logger;
+    private readonly IUncertaintyScoreCalculator _uncertaintyCalculator;
+    private readonly TrustScoreAggregator _trustAggregator;
+    private readonly TimeProvider _timeProvider;
+
+    // Default prior values for signals when simulating (moderate/neutral assumptions)
+    private static readonly IReadOnlyDictionary<string, double> DefaultPriors = new Dictionary<string, double>
+    {
+        ["VEX"] = 0.5,           // Neutral: under_investigation
+        ["EPSS"] = 0.3,          // Below median EPSS score
+        ["Reachability"] = 0.5,  // Unknown reachability
+        ["Runtime"] = 0.3,       // Likely not detected at runtime
+        ["Backport"] = 0.5,      // Unknown backport status
+        ["SBOMLineage"] = 0.5    // Neutral lineage contribution
+    };
+
+    public DeltaIfPresentCalculator(
+        ILogger<DeltaIfPresentCalculator> logger,
+        IUncertaintyScoreCalculator uncertaintyCalculator,
+        TrustScoreAggregator trustAggregator,
+        TimeProvider? timeProvider = null)
+    {
+        _logger = logger;
+        _uncertaintyCalculator = uncertaintyCalculator;
+        _trustAggregator = trustAggregator;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+    }
+
+    public DeltaIfPresentResult CalculateSingleSignalDelta(
+        SignalSnapshot snapshot,
+        string signal,
+        double assumedValue,
+        SignalWeights? weights = null)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+        ArgumentException.ThrowIfNullOrWhiteSpace(signal);
+
+        var effectiveWeights = weights ?? SignalWeights.Default;
+        var signalWeight = GetSignalWeight(signal, effectiveWeights);
+
+        // Calculate current state
+        var currentUncertainty = _uncertaintyCalculator.Calculate(snapshot, effectiveWeights);
+        var currentScore = _trustAggregator.Aggregate(snapshot, currentUncertainty, effectiveWeights);
+
+        // Create hypothetical snapshot with the signal present
+        var hypotheticalSnapshot = CreateHypotheticalSnapshot(snapshot, signal, assumedValue);
+
+        // Calculate hypothetical state
+        var hypotheticalUncertainty = _uncertaintyCalculator.Calculate(hypotheticalSnapshot, effectiveWeights);
+        var hypotheticalScore = _trustAggregator.Aggregate(hypotheticalSnapshot, hypotheticalUncertainty, effectiveWeights);
+
+        DeltaCalculationsCounter.Add(1,
+            new KeyValuePair<string, object?>("signal", signal),
+            new KeyValuePair<string, object?>("cve", snapshot.Cve));
+
+        _logger.LogDebug(
+            "Delta-if-present for {Signal}={Value:F2}: score {Current:F4} -> {Hypothetical:F4} (delta={Delta:+0.0000;-0.0000})",
+            signal, assumedValue, currentScore, hypotheticalScore, hypotheticalScore - currentScore);
+
+        return new DeltaIfPresentResult
+        {
+            Signal = signal,
+            CurrentScore = currentScore,
+            HypotheticalScore = hypotheticalScore,
+            AssumedValue = assumedValue,
+            SignalWeight = signalWeight,
+            CurrentEntropy = currentUncertainty.Entropy,
+            HypotheticalEntropy = hypotheticalUncertainty.Entropy
+        };
+    }
+
+    public DeltaIfPresentAnalysis CalculateFullAnalysis(
+        SignalSnapshot snapshot,
+        SignalWeights? weights = null)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+
+        var effectiveWeights = weights ?? SignalWeights.Default;
+
+        // Calculate current state
+        var currentUncertainty = _uncertaintyCalculator.Calculate(snapshot, effectiveWeights);
+        var currentScore = _trustAggregator.Aggregate(snapshot, currentUncertainty, effectiveWeights);
+
+        var gapAnalysis = new List<SignalDeltaScenarios>();
+
+        // Analyze each gap
+        foreach (var gap in currentUncertainty.Gaps)
+        {
+            var priorValue = DefaultPriors.GetValueOrDefault(gap.Signal, 0.5);
+
+            var bestCase = CalculateSingleSignalDelta(snapshot, gap.Signal, 0.0, effectiveWeights);
+            var worstCase = CalculateSingleSignalDelta(snapshot, gap.Signal, 1.0, effectiveWeights);
+            var priorCase = CalculateSingleSignalDelta(snapshot, gap.Signal, priorValue, effectiveWeights);
+
+            gapAnalysis.Add(new SignalDeltaScenarios
+            {
+                Signal = gap.Signal,
+                Weight = gap.Weight,
+                GapReason = gap.Reason,
+                BestCase = bestCase,
+                WorstCase = worstCase,
+                PriorCase = priorCase
+            });
+        }
+
+        // Prioritize gaps by maximum potential impact
+        var prioritized = gapAnalysis
+            .OrderByDescending(g => g.MaxImpact)
+            .Select(g => g.Signal)
+            .ToList();
+
+        _logger.LogInformation(
+            "Delta-if-present analysis for {Cve}/{Purl}: {GapCount} gaps, prioritized: [{Priority}]",
+            snapshot.Cve, snapshot.Purl, gapAnalysis.Count,
+            string.Join(", ", prioritized.Take(3)));
+
+        return new DeltaIfPresentAnalysis
+        {
+            CurrentScore = currentScore,
+            CurrentEntropy = currentUncertainty.Entropy,
+            GapAnalysis = gapAnalysis,
+            PrioritizedGaps = prioritized,
+            ComputedAt = _timeProvider.GetUtcNow()
+        };
+    }
+
+    public ScoreBounds CalculateScoreBounds(
+        SignalSnapshot snapshot,
+        SignalWeights? weights = null)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+
+        var effectiveWeights = weights ?? SignalWeights.Default;
+
+        // Calculate current state
+        var currentUncertainty = _uncertaintyCalculator.Calculate(snapshot, effectiveWeights);
+        var currentScore = _trustAggregator.Aggregate(snapshot, currentUncertainty, effectiveWeights);
+
+        if (currentUncertainty.Gaps.Count == 0)
+        {
+            // No gaps - current score is the only possibility
+            return new ScoreBounds
+            {
+                CurrentScore = currentScore,
+                MinimumScore = currentScore,
+                MaximumScore = currentScore,
+                CurrentEntropy = currentUncertainty.Entropy,
+                GapCount = 0,
+                MissingWeightPercentage = 0.0
+            };
+        }
+
+        // Create best-case snapshot (all missing signals at low-risk values)
+        var bestSnapshot = snapshot;
+        foreach (var gap in currentUncertainty.Gaps)
+        {
+            bestSnapshot = CreateHypotheticalSnapshot(bestSnapshot, gap.Signal, 0.0);
+        }
+
+        // Create worst-case snapshot (all missing signals at high-risk values)
+        var worstSnapshot = snapshot;
+        foreach (var gap in currentUncertainty.Gaps)
+        {
+            worstSnapshot = CreateHypotheticalSnapshot(worstSnapshot, gap.Signal, 1.0);
+        }
+
+        // Calculate bounds
+        var bestUncertainty = _uncertaintyCalculator.Calculate(bestSnapshot, effectiveWeights);
+        var worstUncertainty = _uncertaintyCalculator.Calculate(worstSnapshot, effectiveWeights);
+
+        var maxScore = _trustAggregator.Aggregate(bestSnapshot, bestUncertainty, effectiveWeights);
+        var minScore = _trustAggregator.Aggregate(worstSnapshot, worstUncertainty, effectiveWeights);
+
+        // Calculate missing weight percentage
+        var missingWeight = currentUncertainty.Gaps.Sum(g => g.Weight);
+        var totalWeight = effectiveWeights.TotalWeight;
+        var missingPercentage = totalWeight > 0 ? (missingWeight / totalWeight) * 100.0 : 0.0;
+
+        _logger.LogDebug(
+            "Score bounds for {Cve}: current={Current:F4}, min={Min:F4}, max={Max:F4}, range={Range:F4}",
+            snapshot.Cve, currentScore, minScore, maxScore, maxScore - minScore);
+
+        return new ScoreBounds
+        {
+            CurrentScore = currentScore,
+            MinimumScore = minScore,
+            MaximumScore = maxScore,
+            CurrentEntropy = currentUncertainty.Entropy,
+            GapCount = currentUncertainty.Gaps.Count,
+            MissingWeightPercentage = missingPercentage
+        };
+    }
+
+    private static double GetSignalWeight(string signal, SignalWeights weights)
+    {
+        return signal.ToUpperInvariant() switch
+        {
+            "VEX" => weights.VexWeight,
+            "EPSS" => weights.EpssWeight,
+            "REACHABILITY" => weights.ReachabilityWeight,
+            "RUNTIME" => weights.RuntimeWeight,
+            "BACKPORT" => weights.BackportWeight,
+            "SBOMLINEAGE" or "SBOM" => weights.SbomLineageWeight,
+            _ => 0.0
+        };
+    }
+
+    private SignalSnapshot CreateHypotheticalSnapshot(
+        SignalSnapshot original,
+        string signal,
+        double normalizedValue)
+    {
+        var now = _timeProvider.GetUtcNow();
+
+        return signal.ToUpperInvariant() switch
+        {
+            "VEX" => original with
+            {
+                Vex = SignalState<VexClaimSummary>.Queried(
+                    CreateHypotheticalVex(normalizedValue), now)
+            },
+            "EPSS" => original with
+            {
+                Epss = SignalState<EpssEvidence>.Queried(
+                    CreateHypotheticalEpss(normalizedValue), now)
+            },
+            "REACHABILITY" => original with
+            {
+                Reachability = SignalState<ReachabilityEvidence>.Queried(
+                    CreateHypotheticalReachability(normalizedValue), now)
+            },
+            "RUNTIME" => original with
+            {
+                Runtime = SignalState<RuntimeEvidence>.Queried(
+                    CreateHypotheticalRuntime(normalizedValue), now)
+            },
+            "BACKPORT" => original with
+            {
+                Backport = SignalState<BackportEvidence>.Queried(
+                    CreateHypotheticalBackport(normalizedValue), now)
+            },
+            "SBOMLINEAGE" or "SBOM" => original with
+            {
+                Sbom = SignalState<SbomLineageEvidence>.Queried(
+                    CreateHypotheticalSbom(normalizedValue), now)
+            },
+            _ => original
+        };
+    }
+
+    private static VexClaimSummary CreateHypotheticalVex(double normalizedValue)
+    {
+        // Map 0.0-1.0 to VEX status
+        var status = normalizedValue switch
+        {
+            < 0.25 => "not_affected",
+            < 0.50 => "under_investigation",
+            < 0.75 => "under_investigation",
+            _ => "affected"
+        };
+
+        return new VexClaimSummary
+        {
+            Status = status,
+            Source = "hypothetical",
+            DocumentId = "delta-if-present-simulation",
+            Timestamp = DateTimeOffset.UtcNow
+        };
+    }
+
+    private static EpssEvidence CreateHypotheticalEpss(double normalizedValue)
+    {
+        return new EpssEvidence
+        {
+            Epss = normalizedValue,
+            Percentile = normalizedValue * 100.0,
+            Date = DateOnly.FromDateTime(DateTime.UtcNow)
+        };
+    }
+
+    private static ReachabilityEvidence CreateHypotheticalReachability(double normalizedValue)
+    {
+        var status = normalizedValue >= 0.5
+            ? ReachabilityStatus.Reachable
+            : ReachabilityStatus.Unreachable;
+
+        return new ReachabilityEvidence
+        {
+            Status = status,
+            Confidence = 1.0 - Math.Abs(normalizedValue - 0.5) * 2,
+            PathCount = normalizedValue >= 0.5 ? 1 : 0,
+            Source = "hypothetical"
+        };
+    }
+
+    private static RuntimeEvidence CreateHypotheticalRuntime(double normalizedValue)
+    {
+        return new RuntimeEvidence
+        {
+            Detected = normalizedValue >= 0.5,
+            Source = "hypothetical",
+            Timestamp = DateTimeOffset.UtcNow
+        };
+    }
+
+    private static BackportEvidence CreateHypotheticalBackport(double normalizedValue)
+    {
+        return new BackportEvidence
+        {
+            Detected = normalizedValue < 0.5, // Backport = lower risk
+            Source = "hypothetical",
+            Timestamp = DateTimeOffset.UtcNow
+        };
+    }
+
+    private static SbomLineageEvidence CreateHypotheticalSbom(double normalizedValue)
+    {
+        return new SbomLineageEvidence
+        {
+            Present = true,
+            Depth = (int)(normalizedValue * 5),
+            Source = "hypothetical"
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/EwsCalculator.cs

@@ -0,0 +1,192 @@
+// -----------------------------------------------------------------------------
+// EwsCalculator.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Unified Evidence-Weighted Score calculator implementation.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Diagnostics.Metrics;
+using Microsoft.Extensions.Logging;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Unified calculator for Evidence-Weighted Scores (EWS).
+/// Orchestrates 6-dimension normalization, weighting, and guardrails.
+/// </summary>
+public sealed class EwsCalculator : IEwsCalculator
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization.EWS");
+    private static readonly Histogram<int> EwsScoreHistogram = Meter.CreateHistogram<int>(
+        "stellaops_ews_score",
+        unit: "score",
+        description: "Evidence-Weighted Score distribution (0-100)");
+    private static readonly Counter<int> GuardrailsAppliedCounter = Meter.CreateCounter<int>(
+        "stellaops_ews_guardrails_applied",
+        description: "Count of guardrails applied to EWS scores");
+
+    private readonly ImmutableDictionary<EwsDimension, IEwsDimensionNormalizer> _normalizers;
+    private readonly IGuardrailsEngine _guardrailsEngine;
+    private readonly TimeProvider _timeProvider;
+    private readonly ILogger<EwsCalculator> _logger;
+
+    public EwsCalculator(
+        IEnumerable<IEwsDimensionNormalizer> normalizers,
+        IGuardrailsEngine guardrailsEngine,
+        TimeProvider? timeProvider = null,
+        ILogger<EwsCalculator>? logger = null)
+    {
+        _normalizers = normalizers.ToImmutableDictionary(n => n.Dimension);
+        _guardrailsEngine = guardrailsEngine;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _logger = logger ?? Microsoft.Extensions.Logging.Abstractions.NullLogger<EwsCalculator>.Instance;
+
+        ValidateNormalizers();
+    }
+
+    /// <summary>
+    /// Creates a default EwsCalculator with all standard normalizers.
+    /// </summary>
+    public static EwsCalculator CreateDefault(
+        TimeProvider? timeProvider = null,
+        ILogger<EwsCalculator>? logger = null)
+    {
+        var normalizers = new IEwsDimensionNormalizer[]
+        {
+            new ReachabilityNormalizer(),
+            new RuntimeSignalsNormalizer(),
+            new BackportEvidenceNormalizer(),
+            new ExploitabilityNormalizer(),
+            new SourceConfidenceNormalizer(),
+            new MitigationStatusNormalizer()
+        };
+
+        return new EwsCalculator(
+            normalizers,
+            new GuardrailsEngine(),
+            timeProvider,
+            logger);
+    }
+
+    /// <inheritdoc />
+    public EwsCompositeScore Calculate(
+        EwsSignalInput signal,
+        EwsDimensionWeights? weights = null,
+        EwsGuardrails? guardrails = null)
+    {
+        ArgumentNullException.ThrowIfNull(signal);
+
+        var effectiveWeights = weights ?? EwsDimensionWeights.Default;
+        var effectiveGuardrails = guardrails ?? EwsGuardrails.Default;
+
+        // Validate weights
+        if (!effectiveWeights.IsNormalized())
+        {
+            _logger.LogWarning(
+                "EWS dimension weights are not normalized (total={Total:F4}); results may be unexpected",
+                effectiveWeights.TotalWeight);
+        }
+
+        // Calculate all dimension scores
+        var dimensionScores = new List<EwsDimensionScore>();
+        foreach (EwsDimension dimension in Enum.GetValues<EwsDimension>())
+        {
+            var dimScore = CalculateDimension(dimension, signal, effectiveWeights.GetWeight(dimension));
+            dimensionScores.Add(dimScore);
+        }
+
+        var dimensions = dimensionScores.ToImmutableArray();
+
+        // Calculate raw composite score (weighted sum)
+        var rawScore = (int)Math.Round(dimensions.Sum(d => d.WeightedContribution));
+        rawScore = Math.Clamp(rawScore, 0, 100);
+
+        // Apply guardrails
+        var guardrailsResult = _guardrailsEngine.Apply(rawScore, signal, dimensions, effectiveGuardrails);
+
+        // Calculate overall confidence (weighted average)
+        var confidence = dimensions.Sum(d => d.Confidence * d.Weight);
+
+        // Determine if manual review is needed
+        var needsReview = confidence < effectiveGuardrails.MinConfidenceThreshold;
+
+        var result = new EwsCompositeScore
+        {
+            Score = guardrailsResult.AdjustedScore,
+            RawScore = rawScore,
+            Confidence = confidence,
+            Dimensions = dimensions,
+            AppliedGuardrails = guardrailsResult.AppliedGuardrails,
+            NeedsReview = needsReview,
+            CalculatedAt = _timeProvider.GetUtcNow(),
+            CveId = signal.CveId,
+            Purl = signal.Purl
+        };
+
+        // Emit metrics
+        EwsScoreHistogram.Record(result.Score,
+            new KeyValuePair<string, object?>("risk_tier", result.RiskTier),
+            new KeyValuePair<string, object?>("guardrails_applied", guardrailsResult.WasModified));
+
+        if (guardrailsResult.WasModified)
+        {
+            GuardrailsAppliedCounter.Add(guardrailsResult.AppliedGuardrails.Length);
+        }
+
+        _logger.LogDebug(
+            "Calculated EWS: score={Score} (raw={RawScore}), confidence={Confidence:P0}, tier={Tier}, guardrails={Guardrails}",
+            result.Score,
+            result.RawScore,
+            result.Confidence,
+            result.RiskTier,
+            string.Join(",", guardrailsResult.AppliedGuardrails));
+
+        return result;
+    }
+
+    /// <inheritdoc />
+    public EwsDimensionScore CalculateDimension(
+        EwsDimension dimension,
+        EwsSignalInput signal,
+        double weight)
+    {
+        var normalizer = GetNormalizer(dimension);
+
+        var score = normalizer.Normalize(signal);
+        var confidence = normalizer.GetConfidence(signal);
+        var explanation = normalizer.GetExplanation(signal, score);
+
+        return new EwsDimensionScore
+        {
+            Dimension = dimension,
+            Score = score,
+            Confidence = confidence,
+            Weight = weight,
+            Explanation = explanation
+        };
+    }
+
+    /// <inheritdoc />
+    public IEwsDimensionNormalizer GetNormalizer(EwsDimension dimension)
+    {
+        if (_normalizers.TryGetValue(dimension, out var normalizer))
+        {
+            return normalizer;
+        }
+
+        throw new InvalidOperationException($"No normalizer registered for dimension {dimension}");
+    }
+
+    private void ValidateNormalizers()
+    {
+        foreach (EwsDimension dimension in Enum.GetValues<EwsDimension>())
+        {
+            if (!_normalizers.ContainsKey(dimension))
+            {
+                throw new InvalidOperationException(
+                    $"Missing normalizer for dimension {dimension}. All 6 dimensions must have normalizers.");
+            }
+        }
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/EwsDimension.cs

@@ -0,0 +1,101 @@
+// -----------------------------------------------------------------------------
+// EwsDimension.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Defines the 6 canonical dimensions for EWS scoring.
+// -----------------------------------------------------------------------------
+
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// The 6 canonical dimensions for Evidence-Weighted Score (EWS) model.
+/// Each dimension maps specific signal inputs to a normalized 0-100 score.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter))]
+public enum EwsDimension
+{
+    /// <summary>
+    /// RCH - Reachability dimension.
+    /// Measures whether vulnerable code paths are reachable from entrypoints.
+    /// Input: Reachability tier (R0-R4), call graph analysis, runtime traces.
+    /// </summary>
+    Reachability = 0,
+
+    /// <summary>
+    /// RTS - Runtime Signals dimension.
+    /// Measures evidence from runtime detection and observability.
+    /// Input: Runtime telemetry, instrumentation coverage, APM signals.
+    /// </summary>
+    RuntimeSignals = 1,
+
+    /// <summary>
+    /// BKP - Backport Evidence dimension.
+    /// Measures evidence of patched code in affected packages.
+    /// Input: Backport detection, binary diff analysis, vendor advisories.
+    /// </summary>
+    BackportEvidence = 2,
+
+    /// <summary>
+    /// XPL - Exploitability dimension.
+    /// Measures likelihood and maturity of exploitation.
+    /// Input: EPSS, KEV status, exploit kit availability, PoC age.
+    /// </summary>
+    Exploitability = 3,
+
+    /// <summary>
+    /// SRC - Source Confidence dimension.
+    /// Measures confidence in SBOM and dependency lineage.
+    /// Input: SBOM completeness, verified signatures, attestations.
+    /// </summary>
+    SourceConfidence = 4,
+
+    /// <summary>
+    /// MIT - Mitigation Status dimension.
+    /// Measures VEX status and compensating controls.
+    /// Input: VEX statements, workarounds applied, network controls.
+    /// </summary>
+    MitigationStatus = 5
+}
+
+/// <summary>
+/// Short codes for dimension serialization and display.
+/// </summary>
+public static class EwsDimensionCodes
+{
+    public const string Reachability = "RCH";
+    public const string RuntimeSignals = "RTS";
+    public const string BackportEvidence = "BKP";
+    public const string Exploitability = "XPL";
+    public const string SourceConfidence = "SRC";
+    public const string MitigationStatus = "MIT";
+
+    /// <summary>
+    /// Gets the short code for a dimension.
+    /// </summary>
+    public static string ToCode(this EwsDimension dimension) => dimension switch
+    {
+        EwsDimension.Reachability => Reachability,
+        EwsDimension.RuntimeSignals => RuntimeSignals,
+        EwsDimension.BackportEvidence => BackportEvidence,
+        EwsDimension.Exploitability => Exploitability,
+        EwsDimension.SourceConfidence => SourceConfidence,
+        EwsDimension.MitigationStatus => MitigationStatus,
+        _ => throw new ArgumentOutOfRangeException(nameof(dimension), dimension, "Unknown dimension")
+    };
+
+    /// <summary>
+    /// Parses a short code to a dimension.
+    /// </summary>
+    public static EwsDimension? FromCode(string code) => code?.ToUpperInvariant() switch
+    {
+        Reachability => EwsDimension.Reachability,
+        RuntimeSignals => EwsDimension.RuntimeSignals,
+        BackportEvidence => EwsDimension.BackportEvidence,
+        Exploitability => EwsDimension.Exploitability,
+        SourceConfidence => EwsDimension.SourceConfidence,
+        MitigationStatus => EwsDimension.MitigationStatus,
+        _ => null
+    };
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/EwsModels.cs

@@ -0,0 +1,298 @@
+// -----------------------------------------------------------------------------
+// EwsModels.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Result models for Evidence-Weighted Score calculation.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Individual dimension score from normalization.
+/// </summary>
+public sealed record EwsDimensionScore
+{
+    /// <summary>
+    /// The dimension this score represents.
+    /// </summary>
+    [JsonPropertyName("dimension")]
+    public required EwsDimension Dimension { get; init; }
+
+    /// <summary>
+    /// Short dimension code (RCH, RTS, BKP, XPL, SRC, MIT).
+    /// </summary>
+    [JsonPropertyName("code")]
+    public string Code => Dimension.ToCode();
+
+    /// <summary>
+    /// Normalized score in range [0, 100].
+    /// </summary>
+    [JsonPropertyName("score")]
+    public required int Score { get; init; }
+
+    /// <summary>
+    /// Confidence level for this score (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("confidence")]
+    public required double Confidence { get; init; }
+
+    /// <summary>
+    /// Weight applied to this dimension in composite calculation.
+    /// </summary>
+    [JsonPropertyName("weight")]
+    public required double Weight { get; init; }
+
+    /// <summary>
+    /// Weighted contribution to composite score.
+    /// </summary>
+    [JsonPropertyName("weighted_contribution")]
+    public double WeightedContribution => Score * Weight;
+
+    /// <summary>
+    /// Human-readable explanation of how the score was derived.
+    /// </summary>
+    [JsonPropertyName("explanation")]
+    public required string Explanation { get; init; }
+
+    /// <summary>
+    /// Whether this score is based on actual evidence or assumptions.
+    /// </summary>
+    [JsonPropertyName("is_evidence_based")]
+    public bool IsEvidenceBased => Confidence >= 0.5;
+}
+
+/// <summary>
+/// Weights for each dimension in the 6-dimension EWS model.
+/// </summary>
+public sealed record EwsDimensionWeights
+{
+    /// <summary>
+    /// Weight for RCH (Reachability) dimension.
+    /// </summary>
+    [JsonPropertyName("rch")]
+    public double Reachability { get; init; } = 0.25;
+
+    /// <summary>
+    /// Weight for RTS (Runtime Signals) dimension.
+    /// </summary>
+    [JsonPropertyName("rts")]
+    public double RuntimeSignals { get; init; } = 0.15;
+
+    /// <summary>
+    /// Weight for BKP (Backport Evidence) dimension.
+    /// </summary>
+    [JsonPropertyName("bkp")]
+    public double BackportEvidence { get; init; } = 0.10;
+
+    /// <summary>
+    /// Weight for XPL (Exploitability) dimension.
+    /// </summary>
+    [JsonPropertyName("xpl")]
+    public double Exploitability { get; init; } = 0.20;
+
+    /// <summary>
+    /// Weight for SRC (Source Confidence) dimension.
+    /// </summary>
+    [JsonPropertyName("src")]
+    public double SourceConfidence { get; init; } = 0.10;
+
+    /// <summary>
+    /// Weight for MIT (Mitigation Status) dimension.
+    /// </summary>
+    [JsonPropertyName("mit")]
+    public double MitigationStatus { get; init; } = 0.20;
+
+    /// <summary>
+    /// Default weights as per advisory recommendations.
+    /// </summary>
+    public static EwsDimensionWeights Default => new();
+
+    /// <summary>
+    /// Legacy 6-dimension weights for backward compatibility.
+    /// </summary>
+    public static EwsDimensionWeights Legacy => new()
+    {
+        Reachability = 0.20,
+        RuntimeSignals = 0.10,
+        BackportEvidence = 0.15,
+        Exploitability = 0.25,
+        SourceConfidence = 0.10,
+        MitigationStatus = 0.20
+    };
+
+    /// <summary>
+    /// Gets the weight for a specific dimension.
+    /// </summary>
+    public double GetWeight(EwsDimension dimension) => dimension switch
+    {
+        EwsDimension.Reachability => Reachability,
+        EwsDimension.RuntimeSignals => RuntimeSignals,
+        EwsDimension.BackportEvidence => BackportEvidence,
+        EwsDimension.Exploitability => Exploitability,
+        EwsDimension.SourceConfidence => SourceConfidence,
+        EwsDimension.MitigationStatus => MitigationStatus,
+        _ => 0.0
+    };
+
+    /// <summary>
+    /// Sum of all weights (should equal 1.0 for normalized calculations).
+    /// </summary>
+    public double TotalWeight =>
+        Reachability + RuntimeSignals + BackportEvidence +
+        Exploitability + SourceConfidence + MitigationStatus;
+
+    /// <summary>
+    /// Validates that weights sum to approximately 1.0.
+    /// </summary>
+    public bool IsNormalized(double tolerance = 0.001) =>
+        Math.Abs(TotalWeight - 1.0) < tolerance;
+}
+
+/// <summary>
+/// Guardrails configuration for EWS scoring.
+/// Defines caps and floors to prevent extreme scores.
+/// </summary>
+public sealed record EwsGuardrails
+{
+    /// <summary>
+    /// Maximum score for "not_affected" VEX status (cap).
+    /// Prevents fully mitigated items from being flagged as high risk.
+    /// </summary>
+    [JsonPropertyName("not_affected_cap")]
+    public int NotAffectedCap { get; init; } = 25;
+
+    /// <summary>
+    /// Minimum score when runtime evidence shows active usage (floor).
+    /// Ensures actively used vulnerable code is never fully suppressed.
+    /// </summary>
+    [JsonPropertyName("runtime_floor")]
+    public int RuntimeFloor { get; init; } = 30;
+
+    /// <summary>
+    /// Maximum score for speculative findings (no evidence, all assumptions).
+    /// Prevents assumption-based findings from dominating triage.
+    /// </summary>
+    [JsonPropertyName("speculative_cap")]
+    public int SpeculativeCap { get; init; } = 60;
+
+    /// <summary>
+    /// Minimum score when CVE is in KEV (floor).
+    /// Known exploited vulnerabilities always require attention.
+    /// </summary>
+    [JsonPropertyName("kev_floor")]
+    public int KevFloor { get; init; } = 70;
+
+    /// <summary>
+    /// Maximum score for backported findings (cap).
+    /// Confirmed backports should not be high priority.
+    /// </summary>
+    [JsonPropertyName("backported_cap")]
+    public int BackportedCap { get; init; } = 20;
+
+    /// <summary>
+    /// Minimum overall confidence to trust the composite score.
+    /// Below this, the score should be flagged for manual review.
+    /// </summary>
+    [JsonPropertyName("min_confidence_threshold")]
+    public double MinConfidenceThreshold { get; init; } = 0.3;
+
+    /// <summary>
+    /// Default guardrails configuration.
+    /// </summary>
+    public static EwsGuardrails Default => new();
+}
+
+/// <summary>
+/// Composite Evidence-Weighted Score result.
+/// </summary>
+public sealed record EwsCompositeScore
+{
+    /// <summary>
+    /// Final weighted composite score [0, 100].
+    /// </summary>
+    [JsonPropertyName("score")]
+    public required int Score { get; init; }
+
+    /// <summary>
+    /// Score before guardrails were applied.
+    /// </summary>
+    [JsonPropertyName("raw_score")]
+    public required int RawScore { get; init; }
+
+    /// <summary>
+    /// Basis points representation (0-10000) for deterministic storage.
+    /// </summary>
+    [JsonPropertyName("basis_points")]
+    public int BasisPoints => Score * 100;
+
+    /// <summary>
+    /// Overall confidence in the composite score (0.0 to 1.0).
+    /// Weighted average of dimension confidences.
+    /// </summary>
+    [JsonPropertyName("confidence")]
+    public required double Confidence { get; init; }
+
+    /// <summary>
+    /// Individual dimension scores.
+    /// </summary>
+    [JsonPropertyName("dimensions")]
+    public required ImmutableArray<EwsDimensionScore> Dimensions { get; init; }
+
+    /// <summary>
+    /// Guardrails that were applied.
+    /// </summary>
+    [JsonPropertyName("applied_guardrails")]
+    public required ImmutableArray<string> AppliedGuardrails { get; init; }
+
+    /// <summary>
+    /// Whether guardrails modified the score.
+    /// </summary>
+    [JsonPropertyName("guardrails_applied")]
+    public bool GuardrailsApplied => Score != RawScore;
+
+    /// <summary>
+    /// Whether manual review is recommended due to low confidence.
+    /// </summary>
+    [JsonPropertyName("needs_review")]
+    public required bool NeedsReview { get; init; }
+
+    /// <summary>
+    /// When this score was calculated (UTC).
+    /// </summary>
+    [JsonPropertyName("calculated_at")]
+    public required DateTimeOffset CalculatedAt { get; init; }
+
+    /// <summary>
+    /// CVE identifier this score relates to.
+    /// </summary>
+    [JsonPropertyName("cve_id")]
+    public string? CveId { get; init; }
+
+    /// <summary>
+    /// Package URL (purl) this score relates to.
+    /// </summary>
+    [JsonPropertyName("purl")]
+    public string? Purl { get; init; }
+
+    /// <summary>
+    /// Gets a dimension score by dimension type.
+    /// </summary>
+    public EwsDimensionScore? GetDimension(EwsDimension dimension) =>
+        Dimensions.FirstOrDefault(d => d.Dimension == dimension);
+
+    /// <summary>
+    /// Gets a risk tier based on the score.
+    /// </summary>
+    [JsonPropertyName("risk_tier")]
+    public string RiskTier => Score switch
+    {
+        >= 80 => "Critical",
+        >= 60 => "High",
+        >= 40 => "Medium",
+        >= 20 => "Low",
+        _ => "Informational"
+    };
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/EwsSignalInput.cs

@@ -0,0 +1,221 @@
+// -----------------------------------------------------------------------------
+// EwsSignalInput.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Signal input model for EWS dimension normalization.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Raw signal inputs for Evidence-Weighted Score calculation.
+/// Contains all signals that feed into the 6-dimension model.
+/// </summary>
+public sealed record EwsSignalInput
+{
+    // -------------------------------------------------------------------------
+    // RCH (Reachability) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// Reachability tier from static analysis (R0=unreachable to R4=reachable).
+    /// </summary>
+    [JsonPropertyName("reachability_tier")]
+    public int? ReachabilityTier { get; init; }
+
+    /// <summary>
+    /// Call graph analysis confidence (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("call_graph_confidence")]
+    public double? CallGraphConfidence { get; init; }
+
+    /// <summary>
+    /// Whether runtime trace confirmed the path.
+    /// </summary>
+    [JsonPropertyName("runtime_trace_confirmed")]
+    public bool? RuntimeTraceConfirmed { get; init; }
+
+    // -------------------------------------------------------------------------
+    // RTS (Runtime Signals) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// Runtime instrumentation coverage percentage (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("instrumentation_coverage")]
+    public double? InstrumentationCoverage { get; init; }
+
+    /// <summary>
+    /// Number of runtime invocations observed in the past period.
+    /// </summary>
+    [JsonPropertyName("runtime_invocation_count")]
+    public int? RuntimeInvocationCount { get; init; }
+
+    /// <summary>
+    /// Whether APM signals indicate active usage.
+    /// </summary>
+    [JsonPropertyName("apm_active_usage")]
+    public bool? ApmActiveUsage { get; init; }
+
+    // -------------------------------------------------------------------------
+    // BKP (Backport Evidence) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// Whether backport was detected via binary analysis.
+    /// </summary>
+    [JsonPropertyName("backport_detected")]
+    public bool? BackportDetected { get; init; }
+
+    /// <summary>
+    /// Backport confidence score from binary diff (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("backport_confidence")]
+    public double? BackportConfidence { get; init; }
+
+    /// <summary>
+    /// Whether vendor advisory confirms backport.
+    /// </summary>
+    [JsonPropertyName("vendor_backport_confirmed")]
+    public bool? VendorBackportConfirmed { get; init; }
+
+    // -------------------------------------------------------------------------
+    // XPL (Exploitability) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// EPSS probability (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("epss_probability")]
+    public double? EpssProbability { get; init; }
+
+    /// <summary>
+    /// Whether the CVE is in KEV (Known Exploited Vulnerabilities).
+    /// </summary>
+    [JsonPropertyName("is_in_kev")]
+    public bool? IsInKev { get; init; }
+
+    /// <summary>
+    /// Whether an exploit kit is available.
+    /// </summary>
+    [JsonPropertyName("exploit_kit_available")]
+    public bool? ExploitKitAvailable { get; init; }
+
+    /// <summary>
+    /// Age of the public PoC in days (null if no PoC).
+    /// </summary>
+    [JsonPropertyName("poc_age_days")]
+    public int? PocAgeDays { get; init; }
+
+    /// <summary>
+    /// CVSS base score (0.0 to 10.0).
+    /// </summary>
+    [JsonPropertyName("cvss_base_score")]
+    public double? CvssBaseScore { get; init; }
+
+    // -------------------------------------------------------------------------
+    // SRC (Source Confidence) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// SBOM completeness percentage (0.0 to 1.0).
+    /// </summary>
+    [JsonPropertyName("sbom_completeness")]
+    public double? SbomCompleteness { get; init; }
+
+    /// <summary>
+    /// Whether SBOM has verified signatures.
+    /// </summary>
+    [JsonPropertyName("sbom_signed")]
+    public bool? SbomSigned { get; init; }
+
+    /// <summary>
+    /// Number of valid attestations.
+    /// </summary>
+    [JsonPropertyName("attestation_count")]
+    public int? AttestationCount { get; init; }
+
+    /// <summary>
+    /// Whether dependency lineage is verified.
+    /// </summary>
+    [JsonPropertyName("lineage_verified")]
+    public bool? LineageVerified { get; init; }
+
+    // -------------------------------------------------------------------------
+    // MIT (Mitigation Status) signals
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// VEX status string (not_affected, affected, fixed, under_investigation).
+    /// </summary>
+    [JsonPropertyName("vex_status")]
+    public string? VexStatus { get; init; }
+
+    /// <summary>
+    /// VEX justification string.
+    /// </summary>
+    [JsonPropertyName("vex_justification")]
+    public string? VexJustification { get; init; }
+
+    /// <summary>
+    /// Whether a workaround is applied.
+    /// </summary>
+    [JsonPropertyName("workaround_applied")]
+    public bool? WorkaroundApplied { get; init; }
+
+    /// <summary>
+    /// Whether network controls mitigate the vulnerability.
+    /// </summary>
+    [JsonPropertyName("network_controls_applied")]
+    public bool? NetworkControlsApplied { get; init; }
+
+    // -------------------------------------------------------------------------
+    // Metadata
+    // -------------------------------------------------------------------------
+
+    /// <summary>
+    /// Timestamp when these signals were collected.
+    /// </summary>
+    [JsonPropertyName("collected_at")]
+    public DateTimeOffset? CollectedAt { get; init; }
+
+    /// <summary>
+    /// CVE identifier this input relates to.
+    /// </summary>
+    [JsonPropertyName("cve_id")]
+    public string? CveId { get; init; }
+
+    /// <summary>
+    /// Package URL (purl) this input relates to.
+    /// </summary>
+    [JsonPropertyName("purl")]
+    public string? Purl { get; init; }
+
+    /// <summary>
+    /// Additional signals as key-value pairs for extensibility.
+    /// </summary>
+    [JsonPropertyName("additional_signals")]
+    public ImmutableDictionary<string, object?>? AdditionalSignals { get; init; }
+
+    /// <summary>
+    /// Creates an empty signal input (all assumptions mode).
+    /// </summary>
+    public static EwsSignalInput Empty => new();
+
+    /// <summary>
+    /// Checks if a signal is present for the specified dimension.
+    /// </summary>
+    public bool HasSignalForDimension(EwsDimension dimension) => dimension switch
+    {
+        EwsDimension.Reachability => ReachabilityTier.HasValue || CallGraphConfidence.HasValue || RuntimeTraceConfirmed.HasValue,
+        EwsDimension.RuntimeSignals => InstrumentationCoverage.HasValue || RuntimeInvocationCount.HasValue || ApmActiveUsage.HasValue,
+        EwsDimension.BackportEvidence => BackportDetected.HasValue || BackportConfidence.HasValue || VendorBackportConfirmed.HasValue,
+        EwsDimension.Exploitability => EpssProbability.HasValue || IsInKev.HasValue || ExploitKitAvailable.HasValue || PocAgeDays.HasValue || CvssBaseScore.HasValue,
+        EwsDimension.SourceConfidence => SbomCompleteness.HasValue || SbomSigned.HasValue || AttestationCount.HasValue || LineageVerified.HasValue,
+        EwsDimension.MitigationStatus => VexStatus != null || WorkaroundApplied.HasValue || NetworkControlsApplied.HasValue,
+        _ => false
+    };
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/GuardrailsEngine.cs

@@ -0,0 +1,109 @@
+// -----------------------------------------------------------------------------
+// GuardrailsEngine.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Implementation of guardrails enforcement for EWS scoring.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Applies guardrails (caps and floors) to EWS scores.
+/// Guardrails prevent extreme scores and ensure business logic constraints.
+/// </summary>
+public sealed class GuardrailsEngine : IGuardrailsEngine
+{
+    /// <inheritdoc />
+    public GuardrailsResult Apply(
+        int rawScore,
+        EwsSignalInput signal,
+        ImmutableArray<EwsDimensionScore> dimensions,
+        EwsGuardrails guardrails)
+    {
+        var score = rawScore;
+        var applied = new List<string>();
+
+        // Check for KEV floor first (highest priority)
+        if (signal.IsInKev == true && score < guardrails.KevFloor)
+        {
+            score = guardrails.KevFloor;
+            applied.Add($"kev_floor:{guardrails.KevFloor}");
+        }
+
+        // Check for backport cap
+        if ((signal.BackportDetected == true || signal.VendorBackportConfirmed == true) 
+            && score > guardrails.BackportedCap)
+        {
+            score = guardrails.BackportedCap;
+            applied.Add($"backported_cap:{guardrails.BackportedCap}");
+        }
+
+        // Check for not_affected cap
+        if (IsNotAffected(signal) && score > guardrails.NotAffectedCap)
+        {
+            score = guardrails.NotAffectedCap;
+            applied.Add($"not_affected_cap:{guardrails.NotAffectedCap}");
+        }
+
+        // Check for runtime floor (if runtime shows active usage)
+        if (HasActiveRuntimeUsage(signal) && score < guardrails.RuntimeFloor)
+        {
+            score = guardrails.RuntimeFloor;
+            applied.Add($"runtime_floor:{guardrails.RuntimeFloor}");
+        }
+
+        // Check for speculative cap (all assumptions, low confidence)
+        if (IsSpeculative(dimensions) && score > guardrails.SpeculativeCap)
+        {
+            score = guardrails.SpeculativeCap;
+            applied.Add($"speculative_cap:{guardrails.SpeculativeCap}");
+        }
+
+        return new GuardrailsResult
+        {
+            AdjustedScore = Math.Clamp(score, 0, 100),
+            OriginalScore = rawScore,
+            AppliedGuardrails = applied.ToImmutableArray()
+        };
+    }
+
+    private static bool IsNotAffected(EwsSignalInput signal)
+    {
+        return signal.VexStatus?.Equals("not_affected", StringComparison.OrdinalIgnoreCase) == true
+            || signal.VexStatus?.Equals("fixed", StringComparison.OrdinalIgnoreCase) == true;
+    }
+
+    private static bool HasActiveRuntimeUsage(EwsSignalInput signal)
+    {
+        return signal.ApmActiveUsage == true 
+            || (signal.RuntimeInvocationCount.HasValue && signal.RuntimeInvocationCount.Value > 0);
+    }
+
+    private static bool IsSpeculative(ImmutableArray<EwsDimensionScore> dimensions)
+    {
+        if (dimensions.IsDefaultOrEmpty)
+        {
+            return true;
+        }
+
+        // Calculate weighted confidence
+        var totalWeight = 0.0;
+        var weightedConfidence = 0.0;
+
+        foreach (var dim in dimensions)
+        {
+            totalWeight += dim.Weight;
+            weightedConfidence += dim.Confidence * dim.Weight;
+        }
+
+        if (totalWeight > 0)
+        {
+            var avgConfidence = weightedConfidence / totalWeight;
+            return avgConfidence < 0.3; // Less than 30% confidence = speculative
+        }
+
+        return true;
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/IEwsCalculator.cs

@@ -0,0 +1,46 @@
+// -----------------------------------------------------------------------------
+// IEwsCalculator.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Interface for the unified Evidence-Weighted Score calculator.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Unified calculator for Evidence-Weighted Scores (EWS).
+/// Orchestrates 6-dimension normalization, weighting, and guardrails.
+/// </summary>
+public interface IEwsCalculator
+{
+    /// <summary>
+    /// Calculates a composite EWS from raw signals.
+    /// </summary>
+    /// <param name="signal">The raw signal input.</param>
+    /// <param name="weights">Optional custom weights (defaults to EwsDimensionWeights.Default).</param>
+    /// <param name="guardrails">Optional guardrails configuration (defaults to EwsGuardrails.Default).</param>
+    /// <returns>The composite EWS result.</returns>
+    EwsCompositeScore Calculate(
+        EwsSignalInput signal,
+        EwsDimensionWeights? weights = null,
+        EwsGuardrails? guardrails = null);
+
+    /// <summary>
+    /// Calculates a single dimension score from raw signals.
+    /// </summary>
+    /// <param name="dimension">The dimension to calculate.</param>
+    /// <param name="signal">The raw signal input.</param>
+    /// <param name="weight">The weight to assign to this dimension.</param>
+    /// <returns>The dimension score.</returns>
+    EwsDimensionScore CalculateDimension(
+        EwsDimension dimension,
+        EwsSignalInput signal,
+        double weight);
+
+    /// <summary>
+    /// Gets the normalizer for a specific dimension.
+    /// </summary>
+    /// <param name="dimension">The dimension.</param>
+    /// <returns>The normalizer for that dimension.</returns>
+    IEwsDimensionNormalizer GetNormalizer(EwsDimension dimension);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/IEwsDimensionNormalizer.cs

@@ -0,0 +1,47 @@
+// -----------------------------------------------------------------------------
+// IEwsDimensionNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Pluggable interface for normalizing signal inputs to dimension scores.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Interface for normalizing raw signal inputs to a canonical 0-100 dimension score.
+/// Each dimension has its own normalizer implementation that handles the specific
+/// signal types and normalization logic for that dimension.
+/// </summary>
+public interface IEwsDimensionNormalizer
+{
+    /// <summary>
+    /// The dimension this normalizer handles.
+    /// </summary>
+    EwsDimension Dimension { get; }
+
+    /// <summary>
+    /// Normalizes a raw signal value to a dimension score in range [0, 100].
+    /// </summary>
+    /// <param name="signal">The raw signal input for this dimension.</param>
+    /// <returns>Normalized score in range [0, 100], where:
+    /// - 0 = lowest risk/impact (e.g., unreachable, fully mitigated)
+    /// - 100 = highest risk/impact (e.g., reachable, actively exploited)
+    /// </returns>
+    int Normalize(EwsSignalInput signal);
+
+    /// <summary>
+    /// Gets the confidence level for this normalization (0.0 to 1.0).
+    /// Lower confidence when assumptions are made or data is missing.
+    /// </summary>
+    /// <param name="signal">The raw signal input for this dimension.</param>
+    /// <returns>Confidence level from 0.0 (all assumptions) to 1.0 (verified evidence).</returns>
+    double GetConfidence(EwsSignalInput signal);
+
+    /// <summary>
+    /// Gets a human-readable explanation of how the score was derived.
+    /// </summary>
+    /// <param name="signal">The raw signal input for this dimension.</param>
+    /// <param name="normalizedScore">The normalized score that was calculated.</param>
+    /// <returns>Explanation suitable for audit and operator review.</returns>
+    string GetExplanation(EwsSignalInput signal, int normalizedScore);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/IGuardrailsEngine.cs

@@ -0,0 +1,57 @@
+// -----------------------------------------------------------------------------
+// IGuardrailsEngine.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Interface for guardrails enforcement in EWS scoring.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Result of guardrails application.
+/// </summary>
+public sealed record GuardrailsResult
+{
+    /// <summary>
+    /// The adjusted score after applying guardrails.
+    /// </summary>
+    public required int AdjustedScore { get; init; }
+
+    /// <summary>
+    /// The original score before guardrails.
+    /// </summary>
+    public required int OriginalScore { get; init; }
+
+    /// <summary>
+    /// List of guardrails that were applied.
+    /// </summary>
+    public required ImmutableArray<string> AppliedGuardrails { get; init; }
+
+    /// <summary>
+    /// Whether the score was modified.
+    /// </summary>
+    public bool WasModified => AdjustedScore != OriginalScore;
+}
+
+/// <summary>
+/// Engine for applying guardrails (caps and floors) to EWS scores.
+/// Guardrails prevent extreme scores in edge cases.
+/// </summary>
+public interface IGuardrailsEngine
+{
+    /// <summary>
+    /// Applies guardrails to a raw composite score.
+    /// </summary>
+    /// <param name="rawScore">The raw composite score before guardrails.</param>
+    /// <param name="signal">The signal input that produced this score.</param>
+    /// <param name="dimensions">The individual dimension scores.</param>
+    /// <param name="guardrails">The guardrails configuration to apply.</param>
+    /// <returns>The result with adjusted score and list of applied guardrails.</returns>
+    GuardrailsResult Apply(
+        int rawScore,
+        EwsSignalInput signal,
+        ImmutableArray<EwsDimensionScore> dimensions,
+        EwsGuardrails guardrails);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/BackportEvidenceNormalizer.cs

@@ -0,0 +1,94 @@
+// -----------------------------------------------------------------------------
+// BackportEvidenceNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for BKP (Backport Evidence) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes backport evidence to the BKP dimension score.
+/// Higher score = more evidence of vulnerability being present (not backported).
+/// Lower score = strong evidence of backport (vulnerability patched).
+/// </summary>
+public sealed class BackportEvidenceNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.BackportEvidence;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        // Vendor confirmation is strongest signal
+        if (signal.VendorBackportConfirmed == true)
+        {
+            return 5; // Almost certainly patched
+        }
+
+        // Binary analysis detected backport
+        if (signal.BackportDetected == true)
+        {
+            if (signal.BackportConfidence.HasValue)
+            {
+                // Lower score = more likely patched
+                return (int)((1.0 - signal.BackportConfidence.Value) * 30);
+            }
+            return 15; // Backport detected with unknown confidence
+        }
+
+        // Binary analysis explicitly found no backport
+        if (signal.BackportDetected == false)
+        {
+            if (signal.BackportConfidence.HasValue)
+            {
+                // Higher confidence in "no backport" = higher risk score
+                return (int)(70 + signal.BackportConfidence.Value * 30);
+            }
+            return 80; // Likely vulnerable
+        }
+
+        // No backport analysis performed - assume vulnerable (conservative)
+        return 75;
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        if (signal.VendorBackportConfirmed.HasValue)
+        {
+            return 0.95; // Vendor confirmation is highly reliable
+        }
+
+        if (signal.BackportDetected.HasValue)
+        {
+            // Use backport confidence if available
+            return signal.BackportConfidence ?? 0.6;
+        }
+
+        return 0.2; // No analysis, low confidence
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        if (signal.VendorBackportConfirmed == true)
+        {
+            return "Vendor confirmed backport; vulnerability patched in this build";
+        }
+
+        if (signal.BackportDetected == true)
+        {
+            var conf = signal.BackportConfidence?.ToString("P0") ?? "unknown";
+            return $"Binary analysis detected backport with {conf} confidence";
+        }
+
+        if (signal.BackportDetected == false)
+        {
+            var conf = signal.BackportConfidence?.ToString("P0") ?? "unknown";
+            return $"Binary analysis found no backport evidence ({conf} confidence)";
+        }
+
+        return "No backport analysis available; assuming vulnerable";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/ExploitabilityNormalizer.cs

@@ -0,0 +1,152 @@
+// -----------------------------------------------------------------------------
+// ExploitabilityNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for XPL (Exploitability) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes exploitability signals to the XPL dimension score.
+/// Maps EPSS, KEV, exploit availability, and CVSS to a 0-100 score.
+/// </summary>
+public sealed class ExploitabilityNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.Exploitability;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        // KEV is the strongest signal
+        if (signal.IsInKev == true)
+        {
+            return 100; // Known exploited = maximum exploitability
+        }
+
+        var score = 0.0;
+        var weights = 0.0;
+
+        // EPSS probability (most predictive)
+        if (signal.EpssProbability.HasValue)
+        {
+            weights += 0.4;
+            // EPSS is already 0-1, scale to 0-100
+            // Apply slight non-linear scaling to emphasize high-EPSS items
+            var epssScore = Math.Pow(signal.EpssProbability.Value, 0.7) * 100;
+            score += epssScore * 0.4;
+        }
+
+        // Exploit kit availability
+        if (signal.ExploitKitAvailable == true)
+        {
+            weights += 0.25;
+            score += 90 * 0.25; // Very high if exploit kit exists
+        }
+        else if (signal.ExploitKitAvailable == false)
+        {
+            weights += 0.25;
+            score += 20 * 0.25; // Lower if explicitly no kit
+        }
+
+        // PoC age (older PoC = more likely weaponized)
+        if (signal.PocAgeDays.HasValue)
+        {
+            weights += 0.15;
+            var pocScore = signal.PocAgeDays.Value switch
+            {
+                <= 7 => 60,     // Fresh PoC
+                <= 30 => 75,    // 1 month old
+                <= 90 => 85,    // 3 months old
+                <= 365 => 90,   // 1 year old
+                _ => 95         // Very old = likely weaponized
+            };
+            score += pocScore * 0.15;
+        }
+
+        // CVSS base score
+        if (signal.CvssBaseScore.HasValue)
+        {
+            weights += 0.2;
+            // Map 0-10 to 0-100
+            score += signal.CvssBaseScore.Value * 10 * 0.2;
+        }
+
+        if (weights > 0)
+        {
+            return (int)Math.Round(score / weights);
+        }
+
+        // No signals - default to moderate exploitability based on CVSS if available
+        return 50;
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        if (signal.IsInKev == true)
+        {
+            return 1.0; // Absolute certainty
+        }
+
+        var confidence = 0.0;
+
+        if (signal.EpssProbability.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.85);
+        }
+
+        if (signal.ExploitKitAvailable.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.9);
+        }
+
+        if (signal.PocAgeDays.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.7);
+        }
+
+        if (signal.CvssBaseScore.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.5); // CVSS alone is less predictive
+        }
+
+        return confidence > 0 ? confidence : 0.3; // Low confidence if no data
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        var parts = new List<string>();
+
+        if (signal.IsInKev == true)
+        {
+            parts.Add("CVE is in CISA KEV (Known Exploited Vulnerabilities)");
+        }
+
+        if (signal.EpssProbability.HasValue)
+        {
+            parts.Add($"EPSS probability {signal.EpssProbability.Value:P2}");
+        }
+
+        if (signal.ExploitKitAvailable == true)
+        {
+            parts.Add("exploit kit available");
+        }
+
+        if (signal.PocAgeDays.HasValue)
+        {
+            parts.Add($"PoC available for {signal.PocAgeDays.Value} days");
+        }
+
+        if (signal.CvssBaseScore.HasValue)
+        {
+            parts.Add($"CVSS base score {signal.CvssBaseScore.Value:F1}");
+        }
+
+        return parts.Count > 0
+            ? string.Join(", ", parts)
+            : "No exploitability signals; assuming moderate risk";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/MitigationStatusNormalizer.cs

@@ -0,0 +1,118 @@
+// -----------------------------------------------------------------------------
+// MitigationStatusNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for MIT (Mitigation Status) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes mitigation status signals to the MIT dimension score.
+/// Lower score = strong mitigation in place (low residual risk).
+/// Higher score = no mitigation or vulnerable status.
+/// </summary>
+public sealed class MitigationStatusNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.MitigationStatus;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        // VEX status is the primary signal
+        var baseScore = ParseVexStatus(signal.VexStatus);
+
+        // Adjust for workarounds
+        if (signal.WorkaroundApplied == true)
+        {
+            baseScore = Math.Max(0, baseScore - 30);
+        }
+
+        // Adjust for network controls
+        if (signal.NetworkControlsApplied == true)
+        {
+            baseScore = Math.Max(0, baseScore - 20);
+        }
+
+        return Math.Clamp(baseScore, 0, 100);
+    }
+
+    private static int ParseVexStatus(string? vexStatus)
+    {
+        return vexStatus?.ToLowerInvariant() switch
+        {
+            "not_affected" => 5,         // Confirmed not affected
+            "fixed" => 10,               // Fix applied
+            "under_investigation" => 60, // Unknown yet
+            "affected" => 90,            // Confirmed vulnerable
+            "exploitable" => 100,        // Actively exploitable
+            null => 75,                  // No VEX = assume affected
+            _ => 75                      // Unknown status = assume affected
+        };
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        if (!string.IsNullOrEmpty(signal.VexStatus))
+        {
+            // VEX status provides good confidence
+            var conf = signal.VexStatus.ToLowerInvariant() switch
+            {
+                "not_affected" => 0.9,
+                "fixed" => 0.85,
+                "affected" => 0.85,
+                "exploitable" => 0.95,
+                "under_investigation" => 0.4,
+                _ => 0.5
+            };
+
+            // Boost confidence if we also have justification
+            if (!string.IsNullOrEmpty(signal.VexJustification))
+            {
+                conf = Math.Min(1.0, conf + 0.1);
+            }
+
+            return conf;
+        }
+
+        // No VEX but have compensating controls
+        if (signal.WorkaroundApplied.HasValue || signal.NetworkControlsApplied.HasValue)
+        {
+            return 0.6;
+        }
+
+        return 0.2; // No mitigation data
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        var parts = new List<string>();
+
+        if (!string.IsNullOrEmpty(signal.VexStatus))
+        {
+            parts.Add($"VEX status: {signal.VexStatus}");
+
+            if (!string.IsNullOrEmpty(signal.VexJustification))
+            {
+                parts.Add($"justification: {signal.VexJustification}");
+            }
+        }
+
+        if (signal.WorkaroundApplied == true)
+        {
+            parts.Add("workaround applied");
+        }
+
+        if (signal.NetworkControlsApplied == true)
+        {
+            parts.Add("network controls in place");
+        }
+
+        return parts.Count > 0
+            ? string.Join(", ", parts)
+            : "No mitigation status available; assuming affected";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/ReachabilityNormalizer.cs

@@ -0,0 +1,122 @@
+// -----------------------------------------------------------------------------
+// ReachabilityNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for RCH (Reachability) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes reachability signals to the RCH dimension score.
+/// Maps R0-R4 tiers and call graph confidence to a 0-100 score.
+/// </summary>
+public sealed class ReachabilityNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.Reachability;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        // Reachability tier takes precedence
+        if (signal.ReachabilityTier.HasValue)
+        {
+            var tierScore = signal.ReachabilityTier.Value switch
+            {
+                0 => 0,   // R0: Unreachable
+                1 => 20,  // R1: Present in dependency but not imported
+                2 => 40,  // R2: Imported but not called
+                3 => 70,  // R3: Called but not reachable from entrypoint
+                4 => 100, // R4: Reachable from entrypoint
+                _ => 50   // Unknown tier - moderate assumption
+            };
+
+            // Adjust by call graph confidence if available
+            if (signal.CallGraphConfidence.HasValue)
+            {
+                // Higher confidence = trust the tier more
+                // Lower confidence = pull toward middle (50)
+                var confidence = signal.CallGraphConfidence.Value;
+                tierScore = (int)(tierScore * confidence + 50 * (1 - confidence));
+            }
+
+            // Runtime trace confirmation boosts the score if reachable
+            if (signal.RuntimeTraceConfirmed == true && tierScore >= 70)
+            {
+                tierScore = Math.Min(100, tierScore + 15);
+            }
+
+            return Math.Clamp(tierScore, 0, 100);
+        }
+
+        // Fall back to call graph confidence only
+        if (signal.CallGraphConfidence.HasValue)
+        {
+            // High confidence but no tier = assume moderate reachability
+            return (int)(50 * signal.CallGraphConfidence.Value) + 25;
+        }
+
+        // Runtime trace only
+        if (signal.RuntimeTraceConfirmed == true)
+        {
+            return 85; // Strong evidence of reachability
+        }
+
+        // No signals - assume reachable (conservative)
+        return 75;
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        if (signal.ReachabilityTier.HasValue)
+        {
+            // Tier with call graph confidence
+            if (signal.CallGraphConfidence.HasValue)
+            {
+                return Math.Min(1.0, 0.7 + signal.CallGraphConfidence.Value * 0.3);
+            }
+            return 0.7; // Tier alone
+        }
+
+        if (signal.CallGraphConfidence.HasValue)
+        {
+            return signal.CallGraphConfidence.Value * 0.6;
+        }
+
+        if (signal.RuntimeTraceConfirmed == true)
+        {
+            return 0.9; // High confidence from runtime
+        }
+
+        return 0.2; // No evidence, pure assumption
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        if (signal.ReachabilityTier.HasValue)
+        {
+            var tierName = signal.ReachabilityTier.Value switch
+            {
+                0 => "unreachable",
+                1 => "in-dependency-not-imported",
+                2 => "imported-not-called",
+                3 => "called-not-entrypoint-reachable",
+                4 => "entrypoint-reachable",
+                _ => "unknown-tier"
+            };
+
+            var confidence = signal.CallGraphConfidence?.ToString("P0") ?? "unknown";
+            return $"Reachability tier R{signal.ReachabilityTier.Value} ({tierName}), call graph confidence {confidence}";
+        }
+
+        if (signal.RuntimeTraceConfirmed == true)
+        {
+            return "Runtime trace confirmed reachability";
+        }
+
+        return "No reachability analysis; assumed reachable (conservative)";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/RuntimeSignalsNormalizer.cs

@@ -0,0 +1,116 @@
+// -----------------------------------------------------------------------------
+// RuntimeSignalsNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for RTS (Runtime Signals) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes runtime signals to the RTS dimension score.
+/// Higher score = more evidence of runtime activity.
+/// </summary>
+public sealed class RuntimeSignalsNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.RuntimeSignals;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        var score = 0.0;
+        var weights = 0.0;
+
+        // Instrumentation coverage
+        if (signal.InstrumentationCoverage.HasValue)
+        {
+            // Higher coverage = more confidence in runtime data
+            // If coverage is high but no invocations, that's good (not used)
+            // If coverage is low, we can't trust the data
+            weights += 0.3;
+            score += signal.InstrumentationCoverage.Value * 0.3;
+        }
+
+        // Runtime invocation count
+        if (signal.RuntimeInvocationCount.HasValue)
+        {
+            weights += 0.4;
+            // Logarithmic scale for invocations
+            // 0 = 0, 1-10 = 25, 11-100 = 50, 101-1000 = 75, 1000+ = 100
+            var invScore = signal.RuntimeInvocationCount.Value switch
+            {
+                0 => 0.0,
+                <= 10 => 0.25,
+                <= 100 => 0.5,
+                <= 1000 => 0.75,
+                _ => 1.0
+            };
+            score += invScore * 0.4;
+        }
+
+        // APM active usage
+        if (signal.ApmActiveUsage.HasValue)
+        {
+            weights += 0.3;
+            score += (signal.ApmActiveUsage.Value ? 1.0 : 0.0) * 0.3;
+        }
+
+        if (weights > 0)
+        {
+            return (int)Math.Round(score / weights * 100);
+        }
+
+        // No runtime signals - assume moderate risk (we don't know)
+        return 50;
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        var confidence = 0.0;
+
+        if (signal.InstrumentationCoverage.HasValue)
+        {
+            // Coverage itself tells us confidence
+            confidence = Math.Max(confidence, signal.InstrumentationCoverage.Value);
+        }
+
+        if (signal.RuntimeInvocationCount.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.8); // Good data point
+        }
+
+        if (signal.ApmActiveUsage.HasValue)
+        {
+            confidence = Math.Max(confidence, 0.7);
+        }
+
+        return confidence > 0 ? confidence : 0.2; // Low if no data
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        var parts = new List<string>();
+
+        if (signal.InstrumentationCoverage.HasValue)
+        {
+            parts.Add($"instrumentation coverage {signal.InstrumentationCoverage.Value:P0}");
+        }
+
+        if (signal.RuntimeInvocationCount.HasValue)
+        {
+            parts.Add($"{signal.RuntimeInvocationCount.Value} runtime invocations observed");
+        }
+
+        if (signal.ApmActiveUsage.HasValue)
+        {
+            parts.Add(signal.ApmActiveUsage.Value ? "APM shows active usage" : "APM shows no active usage");
+        }
+
+        return parts.Count > 0
+            ? string.Join(", ", parts)
+            : "No runtime signals available; assuming moderate activity";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/EvidenceWeightedScoring/Normalizers/SourceConfidenceNormalizer.cs

@@ -0,0 +1,138 @@
+// -----------------------------------------------------------------------------
+// SourceConfidenceNormalizer.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Normalizer for SRC (Source Confidence) dimension.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+/// <summary>
+/// Normalizes source confidence signals to the SRC dimension score.
+/// Higher score = less confidence in source data (higher uncertainty risk).
+/// Lower score = high confidence in source data.
+/// </summary>
+public sealed class SourceConfidenceNormalizer : IEwsDimensionNormalizer
+{
+    /// <inheritdoc />
+    public EwsDimension Dimension => EwsDimension.SourceConfidence;
+
+    /// <inheritdoc />
+    public int Normalize(EwsSignalInput signal)
+    {
+        // This dimension is inverted: high confidence in source = low risk
+        // We calculate confidence then invert
+
+        var confidenceScore = 0.0;
+        var weights = 0.0;
+
+        // SBOM completeness
+        if (signal.SbomCompleteness.HasValue)
+        {
+            weights += 0.35;
+            confidenceScore += signal.SbomCompleteness.Value * 0.35;
+        }
+
+        // SBOM signed
+        if (signal.SbomSigned.HasValue)
+        {
+            weights += 0.25;
+            confidenceScore += (signal.SbomSigned.Value ? 1.0 : 0.0) * 0.25;
+        }
+
+        // Attestation count
+        if (signal.AttestationCount.HasValue)
+        {
+            weights += 0.2;
+            // More attestations = more confidence (diminishing returns)
+            var attScore = signal.AttestationCount.Value switch
+            {
+                0 => 0.0,
+                1 => 0.5,
+                2 => 0.7,
+                3 => 0.85,
+                _ => 1.0
+            };
+            confidenceScore += attScore * 0.2;
+        }
+
+        // Lineage verified
+        if (signal.LineageVerified.HasValue)
+        {
+            weights += 0.2;
+            confidenceScore += (signal.LineageVerified.Value ? 1.0 : 0.0) * 0.2;
+        }
+
+        if (weights > 0)
+        {
+            var normalizedConfidence = confidenceScore / weights;
+            // Invert: high confidence = low score (low risk from source uncertainty)
+            return (int)Math.Round((1.0 - normalizedConfidence) * 100);
+        }
+
+        // No source signals - assume high uncertainty
+        return 80;
+    }
+
+    /// <inheritdoc />
+    public double GetConfidence(EwsSignalInput signal)
+    {
+        var hasData = signal.SbomCompleteness.HasValue ||
+                      signal.SbomSigned.HasValue ||
+                      signal.AttestationCount.HasValue ||
+                      signal.LineageVerified.HasValue;
+
+        if (!hasData)
+        {
+            return 0.2;
+        }
+
+        // Count how many signals we have
+        var signalCount = 0;
+        if (signal.SbomCompleteness.HasValue) signalCount++;
+        if (signal.SbomSigned.HasValue) signalCount++;
+        if (signal.AttestationCount.HasValue) signalCount++;
+        if (signal.LineageVerified.HasValue) signalCount++;
+
+        // More signals = higher confidence in our assessment
+        return 0.4 + (signalCount * 0.15);
+    }
+
+    /// <inheritdoc />
+    public string GetExplanation(EwsSignalInput signal, int normalizedScore)
+    {
+        var parts = new List<string>();
+
+        if (signal.SbomCompleteness.HasValue)
+        {
+            parts.Add($"SBOM completeness {signal.SbomCompleteness.Value:P0}");
+        }
+
+        if (signal.SbomSigned == true)
+        {
+            parts.Add("SBOM is signed");
+        }
+        else if (signal.SbomSigned == false)
+        {
+            parts.Add("SBOM is not signed");
+        }
+
+        if (signal.AttestationCount.HasValue)
+        {
+            parts.Add($"{signal.AttestationCount.Value} attestation(s) available");
+        }
+
+        if (signal.LineageVerified == true)
+        {
+            parts.Add("dependency lineage verified");
+        }
+        else if (signal.LineageVerified == false)
+        {
+            parts.Add("dependency lineage not verified");
+        }
+
+        return parts.Count > 0
+            ? string.Join(", ", parts)
+            : "No source confidence signals; assuming high uncertainty";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/IDeltaIfPresentCalculator.cs

@@ -0,0 +1,217 @@
+// <copyright file="IDeltaIfPresentCalculator.cs" company="StellaOps">
+// Copyright (c) StellaOps. Licensed under the BUSL-1.1.
+// </copyright>
+
+using StellaOps.Policy.Determinization.Models;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Calculates hypothetical score changes if missing signals were present with various assumed values.
+/// This enables "what-if" analysis to help operators prioritize signal collection efforts.
+/// </summary>
+public interface IDeltaIfPresentCalculator
+{
+    /// <summary>
+    /// Calculate the hypothetical trust score delta if a specific missing signal were present.
+    /// </summary>
+    /// <param name="snapshot">Current signal snapshot.</param>
+    /// <param name="signal">The missing signal to simulate.</param>
+    /// <param name="assumedValue">The assumed value for the signal (0.0-1.0 normalized score).</param>
+    /// <param name="weights">Optional signal weights.</param>
+    /// <returns>Delta calculation result showing impact.</returns>
+    DeltaIfPresentResult CalculateSingleSignalDelta(
+        SignalSnapshot snapshot,
+        string signal,
+        double assumedValue,
+        SignalWeights? weights = null);
+
+    /// <summary>
+    /// Calculate hypothetical impacts for all missing signals at multiple assumed values.
+    /// </summary>
+    /// <param name="snapshot">Current signal snapshot.</param>
+    /// <param name="weights">Optional signal weights.</param>
+    /// <returns>Full delta-if-present analysis for all gaps.</returns>
+    DeltaIfPresentAnalysis CalculateFullAnalysis(
+        SignalSnapshot snapshot,
+        SignalWeights? weights = null);
+
+    /// <summary>
+    /// Calculate the best-case and worst-case score bounds if all missing signals were present.
+    /// </summary>
+    /// <param name="snapshot">Current signal snapshot.</param>
+    /// <param name="weights">Optional signal weights.</param>
+    /// <returns>Score bounds with completeness impact.</returns>
+    ScoreBounds CalculateScoreBounds(
+        SignalSnapshot snapshot,
+        SignalWeights? weights = null);
+}
+
+/// <summary>
+/// Result of a single signal delta-if-present calculation.
+/// </summary>
+public sealed record DeltaIfPresentResult
+{
+    /// <summary>
+    /// The signal that was simulated as present.
+    /// </summary>
+    public required string Signal { get; init; }
+
+    /// <summary>
+    /// The current score without this signal.
+    /// </summary>
+    public required double CurrentScore { get; init; }
+
+    /// <summary>
+    /// The hypothetical score with this signal present at the assumed value.
+    /// </summary>
+    public required double HypotheticalScore { get; init; }
+
+    /// <summary>
+    /// The delta (hypothetical - current). Positive means score would increase.
+    /// </summary>
+    public double Delta => HypotheticalScore - CurrentScore;
+
+    /// <summary>
+    /// The assumed value used for the simulation.
+    /// </summary>
+    public required double AssumedValue { get; init; }
+
+    /// <summary>
+    /// The weight of this signal in the scoring model.
+    /// </summary>
+    public required double SignalWeight { get; init; }
+
+    /// <summary>
+    /// Current entropy before adding signal.
+    /// </summary>
+    public required double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// Hypothetical entropy after adding signal.
+    /// </summary>
+    public required double HypotheticalEntropy { get; init; }
+
+    /// <summary>
+    /// Change in entropy (negative means entropy would decrease = less uncertainty).
+    /// </summary>
+    public double EntropyDelta => HypotheticalEntropy - CurrentEntropy;
+}
+
+/// <summary>
+/// Complete analysis of all missing signals with delta-if-present calculations.
+/// </summary>
+public sealed record DeltaIfPresentAnalysis
+{
+    /// <summary>
+    /// Current aggregate score.
+    /// </summary>
+    public required double CurrentScore { get; init; }
+
+    /// <summary>
+    /// Current entropy (uncertainty).
+    /// </summary>
+    public required double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// List of missing signals with their potential impact at different assumed values.
+    /// </summary>
+    public required IReadOnlyList<SignalDeltaScenarios> GapAnalysis { get; init; }
+
+    /// <summary>
+    /// Prioritized list of signals by maximum potential impact.
+    /// </summary>
+    public required IReadOnlyList<string> PrioritizedGaps { get; init; }
+
+    /// <summary>
+    /// When this analysis was computed.
+    /// </summary>
+    public required DateTimeOffset ComputedAt { get; init; }
+}
+
+/// <summary>
+/// Delta scenarios for a single missing signal at various assumed values.
+/// </summary>
+public sealed record SignalDeltaScenarios
+{
+    /// <summary>
+    /// Signal name.
+    /// </summary>
+    public required string Signal { get; init; }
+
+    /// <summary>
+    /// Signal weight in scoring model.
+    /// </summary>
+    public required double Weight { get; init; }
+
+    /// <summary>
+    /// Why this signal is missing.
+    /// </summary>
+    public required SignalGapReason GapReason { get; init; }
+
+    /// <summary>
+    /// Delta if signal present with best-case value (lowest risk contribution).
+    /// </summary>
+    public required DeltaIfPresentResult BestCase { get; init; }
+
+    /// <summary>
+    /// Delta if signal present with worst-case value (highest risk contribution).
+    /// </summary>
+    public required DeltaIfPresentResult WorstCase { get; init; }
+
+    /// <summary>
+    /// Delta if signal present with prior/expected value.
+    /// </summary>
+    public required DeltaIfPresentResult PriorCase { get; init; }
+
+    /// <summary>
+    /// Maximum absolute delta magnitude across all scenarios.
+    /// </summary>
+    public double MaxImpact => Math.Max(Math.Abs(BestCase.Delta), Math.Abs(WorstCase.Delta));
+}
+
+/// <summary>
+/// Best-case and worst-case score bounds if all missing signals were present.
+/// </summary>
+public sealed record ScoreBounds
+{
+    /// <summary>
+    /// Current score with missing signals.
+    /// </summary>
+    public required double CurrentScore { get; init; }
+
+    /// <summary>
+    /// Minimum possible score (all missing signals at worst-case values).
+    /// </summary>
+    public required double MinimumScore { get; init; }
+
+    /// <summary>
+    /// Maximum possible score (all missing signals at best-case values).
+    /// </summary>
+    public required double MaximumScore { get; init; }
+
+    /// <summary>
+    /// Score range (max - min).
+    /// </summary>
+    public double Range => MaximumScore - MinimumScore;
+
+    /// <summary>
+    /// Current entropy.
+    /// </summary>
+    public required double CurrentEntropy { get; init; }
+
+    /// <summary>
+    /// Entropy if all signals were present (would be 0).
+    /// </summary>
+    public double CompleteEntropy => 0.0;
+
+    /// <summary>
+    /// Number of missing signals.
+    /// </summary>
+    public required int GapCount { get; init; }
+
+    /// <summary>
+    /// Percentage of score weight that is missing.
+    /// </summary>
+    public required double MissingWeightPercentage { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/IImpactScoreCalculator.cs

@@ -0,0 +1,35 @@
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Interface for impact score calculation.
+/// </summary>
+public interface IImpactScoreCalculator
+{
+    /// <summary>
+    /// Calculates the multi-factor impact score for unknowns.
+    /// </summary>
+    /// <param name="context">Impact context with environment, data sensitivity, fleet prevalence, SLA tier, and CVSS.</param>
+    /// <param name="weights">Optional custom weights (uses defaults if null).</param>
+    /// <returns>Calculated impact score with all component scores.</returns>
+    ImpactScore Calculate(ImpactContext context, ImpactFactorWeights? weights = null);
+
+    /// <summary>
+    /// Normalizes an environment type to a score [0.0, 1.0].
+    /// </summary>
+    double NormalizeEnvironment(EnvironmentType environment);
+
+    /// <summary>
+    /// Normalizes a data sensitivity level to a score [0.0, 1.0].
+    /// </summary>
+    double NormalizeDataSensitivity(DataSensitivity sensitivity);
+
+    /// <summary>
+    /// Normalizes an SLA tier to a score [0.0, 1.0].
+    /// </summary>
+    double NormalizeSlaTier(SlaTier tier);
+
+    /// <summary>
+    /// Normalizes a CVSS score [0.0, 10.0] to a score [0.0, 1.0].
+    /// </summary>
+    double NormalizeCvss(double cvssScore);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/ImpactFactorWeights.cs

@@ -0,0 +1,42 @@
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Configurable weights for impact scoring factors.
+/// All weights are normalized to sum to 1.0.
+/// </summary>
+public sealed record ImpactFactorWeights
+{
+    /// <summary>Default weights following advisory recommendations.</summary>
+    public static readonly ImpactFactorWeights Default = new()
+    {
+        EnvironmentExposureWeight = 0.20,
+        DataSensitivityWeight = 0.20,
+        FleetPrevalenceWeight = 0.15,
+        SlaTierWeight = 0.15,
+        CvssSeverityWeight = 0.30
+    };
+
+    /// <summary>Weight for environment exposure factor (prod/stage/dev).</summary>
+    public required double EnvironmentExposureWeight { get; init; }
+
+    /// <summary>Weight for data sensitivity factor (PII, financial, etc.).</summary>
+    public required double DataSensitivityWeight { get; init; }
+
+    /// <summary>Weight for fleet prevalence factor (how many assets affected).</summary>
+    public required double FleetPrevalenceWeight { get; init; }
+
+    /// <summary>Weight for SLA tier factor (business criticality).</summary>
+    public required double SlaTierWeight { get; init; }
+
+    /// <summary>Weight for CVSS severity factor.</summary>
+    public required double CvssSeverityWeight { get; init; }
+
+    /// <summary>Sum of all weights (should equal 1.0 for normalized calculations).</summary>
+    public double TotalWeight =>
+        EnvironmentExposureWeight + DataSensitivityWeight + FleetPrevalenceWeight +
+        SlaTierWeight + CvssSeverityWeight;
+
+    /// <summary>Validates that weights sum to approximately 1.0.</summary>
+    public bool IsNormalized(double tolerance = 0.001) =>
+        Math.Abs(TotalWeight - 1.0) < tolerance;
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/ImpactModels.cs

@@ -0,0 +1,177 @@
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Environment type classification for exposure scoring.
+/// </summary>
+public enum EnvironmentType
+{
+    /// <summary>Development environment - lowest exposure.</summary>
+    Development = 0,
+
+    /// <summary>Testing/QA environment.</summary>
+    Testing = 1,
+
+    /// <summary>Staging/Pre-production environment.</summary>
+    Staging = 2,
+
+    /// <summary>Production environment - highest exposure.</summary>
+    Production = 3
+}
+
+/// <summary>
+/// Data sensitivity classification for impact scoring.
+/// </summary>
+public enum DataSensitivity
+{
+    /// <summary>Public or non-sensitive data.</summary>
+    Public = 0,
+
+    /// <summary>Internal/company-confidential data.</summary>
+    Internal = 1,
+
+    /// <summary>Contains PII (Personally Identifiable Information).</summary>
+    Pii = 2,
+
+    /// <summary>Contains financial data.</summary>
+    Financial = 3,
+
+    /// <summary>Contains healthcare/PHI data.</summary>
+    Healthcare = 4,
+
+    /// <summary>Contains classified/government data.</summary>
+    Classified = 5
+}
+
+/// <summary>
+/// SLA tier for business criticality scoring.
+/// </summary>
+public enum SlaTier
+{
+    /// <summary>Non-critical - can tolerate extended downtime.</summary>
+    NonCritical = 0,
+
+    /// <summary>Standard - normal business operations.</summary>
+    Standard = 1,
+
+    /// <summary>Important - customer-facing or revenue-impacting.</summary>
+    Important = 2,
+
+    /// <summary>Critical - core business functionality.</summary>
+    Critical = 3,
+
+    /// <summary>Mission-critical - business cannot operate without.</summary>
+    MissionCritical = 4
+}
+
+/// <summary>
+/// Input context for impact scoring calculation.
+/// </summary>
+public sealed record ImpactContext
+{
+    /// <summary>Environment where the component is deployed.</summary>
+    [JsonPropertyName("environment")]
+    public required EnvironmentType Environment { get; init; }
+
+    /// <summary>Highest data sensitivity level accessed by the component.</summary>
+    [JsonPropertyName("data_sensitivity")]
+    public required DataSensitivity DataSensitivity { get; init; }
+
+    /// <summary>Proportion of fleet affected (0.0-1.0).</summary>
+    [JsonPropertyName("fleet_prevalence")]
+    public required double FleetPrevalence { get; init; }
+
+    /// <summary>SLA tier of the affected service.</summary>
+    [JsonPropertyName("sla_tier")]
+    public required SlaTier SlaTier { get; init; }
+
+    /// <summary>CVSS base score (0.0-10.0).</summary>
+    [JsonPropertyName("cvss_score")]
+    public required double CvssScore { get; init; }
+
+    /// <summary>
+    /// Creates a default context for unknowns (conservative scoring).
+    /// </summary>
+    public static ImpactContext DefaultForUnknowns() => new()
+    {
+        Environment = EnvironmentType.Production,  // Assume worst-case
+        DataSensitivity = DataSensitivity.Internal, // Conservative default
+        FleetPrevalence = 0.5,                       // Assume moderate prevalence
+        SlaTier = SlaTier.Standard,                  // Standard tier
+        CvssScore = 5.0                              // Medium severity default
+    };
+}
+
+/// <summary>
+/// Result of impact score calculation.
+/// </summary>
+public sealed record ImpactScore
+{
+    /// <summary>Final weighted impact score [0.0, 1.0].</summary>
+    [JsonPropertyName("score")]
+    public required double Score { get; init; }
+
+    /// <summary>Basis points representation (0-10000) for deterministic storage.</summary>
+    [JsonPropertyName("basis_points")]
+    public required int BasisPoints { get; init; }
+
+    /// <summary>Environment exposure component score [0.0, 1.0].</summary>
+    [JsonPropertyName("env_exposure")]
+    public required double EnvironmentExposure { get; init; }
+
+    /// <summary>Data sensitivity component score [0.0, 1.0].</summary>
+    [JsonPropertyName("data_sensitivity")]
+    public required double DataSensitivityScore { get; init; }
+
+    /// <summary>Fleet prevalence component score [0.0, 1.0].</summary>
+    [JsonPropertyName("fleet_prevalence")]
+    public required double FleetPrevalenceScore { get; init; }
+
+    /// <summary>SLA tier component score [0.0, 1.0].</summary>
+    [JsonPropertyName("sla_tier")]
+    public required double SlaTierScore { get; init; }
+
+    /// <summary>CVSS severity component score [0.0, 1.0].</summary>
+    [JsonPropertyName("cvss_severity")]
+    public required double CvssSeverityScore { get; init; }
+
+    /// <summary>When this score was calculated (UTC).</summary>
+    [JsonPropertyName("calculated_at")]
+    public required DateTimeOffset CalculatedAt { get; init; }
+
+    /// <summary>
+    /// Creates an impact score from component scores and weights.
+    /// </summary>
+    public static ImpactScore Create(
+        double envExposure,
+        double dataSensitivity,
+        double fleetPrevalence,
+        double slaTier,
+        double cvssSeverity,
+        ImpactFactorWeights weights,
+        DateTimeOffset calculatedAt)
+    {
+        var score = 
+            envExposure * weights.EnvironmentExposureWeight +
+            dataSensitivity * weights.DataSensitivityWeight +
+            fleetPrevalence * weights.FleetPrevalenceWeight +
+            slaTier * weights.SlaTierWeight +
+            cvssSeverity * weights.CvssSeverityWeight;
+
+        var clampedScore = Math.Clamp(score, 0.0, 1.0);
+        var basisPoints = (int)Math.Round(clampedScore * 10000);
+
+        return new ImpactScore
+        {
+            Score = clampedScore,
+            BasisPoints = basisPoints,
+            EnvironmentExposure = envExposure,
+            DataSensitivityScore = dataSensitivity,
+            FleetPrevalenceScore = fleetPrevalence,
+            SlaTierScore = slaTier,
+            CvssSeverityScore = cvssSeverity,
+            CalculatedAt = calculatedAt
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/ImpactScoreCalculator.cs

@@ -0,0 +1,127 @@
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Calculates multi-factor impact scores for unknowns using the formula:
+/// impact = w_env * EnvExposure + w_data * DataSensitivity + w_fleet * FleetPrevalence + w_sla * SLATier + w_cvss * CVSSSeverity
+/// </summary>
+public sealed class ImpactScoreCalculator : IImpactScoreCalculator
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization");
+    private static readonly Histogram<double> ImpactHistogram = Meter.CreateHistogram<double>(
+        "stellaops_determinization_impact_score",
+        unit: "ratio",
+        description: "Impact score for unknowns (0.0 = minimal impact, 1.0 = critical impact)");
+
+    private readonly ILogger<ImpactScoreCalculator> _logger;
+    private readonly TimeProvider _timeProvider;
+
+    public ImpactScoreCalculator(ILogger<ImpactScoreCalculator> logger, TimeProvider? timeProvider = null)
+    {
+        _logger = logger;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+    }
+
+    /// <inheritdoc />
+    public ImpactScore Calculate(ImpactContext context, ImpactFactorWeights? weights = null)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+
+        var effectiveWeights = weights ?? ImpactFactorWeights.Default;
+
+        // Validate weights
+        if (!effectiveWeights.IsNormalized())
+        {
+            _logger.LogWarning(
+                "Impact factor weights are not normalized (total={Total:F4}); results may be unexpected",
+                effectiveWeights.TotalWeight);
+        }
+
+        // Normalize each dimension
+        var envScore = NormalizeEnvironment(context.Environment);
+        var dataScore = NormalizeDataSensitivity(context.DataSensitivity);
+        var fleetScore = Math.Clamp(context.FleetPrevalence, 0.0, 1.0);
+        var slaScore = NormalizeSlaTier(context.SlaTier);
+        var cvssScore = NormalizeCvss(context.CvssScore);
+
+        // Create result with all components
+        var result = ImpactScore.Create(
+            envScore,
+            dataScore,
+            fleetScore,
+            slaScore,
+            cvssScore,
+            effectiveWeights,
+            _timeProvider.GetUtcNow());
+
+        _logger.LogDebug(
+            "Calculated impact score {Score:F4} (basis points={BasisPoints}) from env={Env:F2}, data={Data:F2}, fleet={Fleet:F2}, sla={Sla:F2}, cvss={Cvss:F2}",
+            result.Score,
+            result.BasisPoints,
+            envScore,
+            dataScore,
+            fleetScore,
+            slaScore,
+            cvssScore);
+
+        // Emit metric
+        ImpactHistogram.Record(result.Score,
+            new KeyValuePair<string, object?>("environment", context.Environment.ToString()),
+            new KeyValuePair<string, object?>("data_sensitivity", context.DataSensitivity.ToString()));
+
+        return result;
+    }
+
+    /// <inheritdoc />
+    public double NormalizeEnvironment(EnvironmentType environment)
+    {
+        // Development = 0.0, Production = 1.0
+        return environment switch
+        {
+            EnvironmentType.Development => 0.0,
+            EnvironmentType.Testing => 0.33,
+            EnvironmentType.Staging => 0.66,
+            EnvironmentType.Production => 1.0,
+            _ => 0.5 // Unknown defaults to moderate
+        };
+    }
+
+    /// <inheritdoc />
+    public double NormalizeDataSensitivity(DataSensitivity sensitivity)
+    {
+        // Public = 0.0, Classified = 1.0
+        return sensitivity switch
+        {
+            DataSensitivity.Public => 0.0,
+            DataSensitivity.Internal => 0.2,
+            DataSensitivity.Pii => 0.5,
+            DataSensitivity.Financial => 0.7,
+            DataSensitivity.Healthcare => 0.8,
+            DataSensitivity.Classified => 1.0,
+            _ => 0.5 // Unknown defaults to moderate
+        };
+    }
+
+    /// <inheritdoc />
+    public double NormalizeSlaTier(SlaTier tier)
+    {
+        // NonCritical = 0.0, MissionCritical = 1.0
+        return tier switch
+        {
+            SlaTier.NonCritical => 0.0,
+            SlaTier.Standard => 0.25,
+            SlaTier.Important => 0.5,
+            SlaTier.Critical => 0.75,
+            SlaTier.MissionCritical => 1.0,
+            _ => 0.5 // Unknown defaults to moderate
+        };
+    }
+
+    /// <inheritdoc />
+    public double NormalizeCvss(double cvssScore)
+    {
+        // CVSS 0.0-10.0 -> 0.0-1.0
+        return Math.Clamp(cvssScore / 10.0, 0.0, 1.0);
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/ScoreV1Predicate.cs

@@ -0,0 +1,160 @@
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Scoring;
+using StellaOps.Policy.TrustLattice;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Score.v1 predicate format for DSSE-signable attestation.
+/// Contains all scoring dimensions in a single, deterministic payload.
+/// All numeric scores use basis points (0-10000) for bit-exact determinism.
+/// </summary>
+public sealed record ScoreV1Predicate
+{
+    /// <summary>
+    /// Predicate type URI for DSSE/In-Toto attestations.
+    /// </summary>
+    public const string PredicateType = "https://stella-ops.org/predicates/score/v1";
+
+    /// <summary>
+    /// Artifact being scored (PURL or component identifier).
+    /// </summary>
+    public required string ArtifactId { get; init; }
+
+    /// <summary>
+    /// Vulnerability identifier if applicable (CVE, GHSA, etc.).
+    /// </summary>
+    public string? VulnerabilityId { get; init; }
+
+    /// <summary>
+    /// Final trust score in basis points (0-10000).
+    /// </summary>
+    public required int TrustScoreBps { get; init; }
+
+    /// <summary>
+    /// Risk tier derived from trust score.
+    /// </summary>
+    public required string Tier { get; init; }
+
+    /// <summary>
+    /// Lattice verdict from K4 logic evaluation.
+    /// </summary>
+    public required K4Value LatticeVerdict { get; init; }
+
+    /// <summary>
+    /// Uncertainty entropy in basis points (0-10000).
+    /// </summary>
+    public required int UncertaintyBps { get; init; }
+
+    /// <summary>
+    /// Individual dimension scores in basis points.
+    /// </summary>
+    public required ScoreDimensionsBps Dimensions { get; init; }
+
+    /// <summary>
+    /// Weights used for this scoring (in basis points).
+    /// </summary>
+    public required WeightsBps WeightsUsed { get; init; }
+
+    /// <summary>
+    /// Policy digest (SHA-256) for reproducibility.
+    /// </summary>
+    public required string PolicyDigest { get; init; }
+
+    /// <summary>
+    /// Timestamp when score was computed (UTC).
+    /// </summary>
+    public required DateTimeOffset ComputedAt { get; init; }
+
+    /// <summary>
+    /// Tenant/namespace scope.
+    /// </summary>
+    public string? TenantId { get; init; }
+}
+
+/// <summary>
+/// Individual scoring dimension values in basis points.
+/// </summary>
+public sealed record ScoreDimensionsBps
+{
+    /// <summary>
+    /// Base severity score (from CVSS or equivalent) in basis points.
+    /// </summary>
+    public required int BaseSeverityBps { get; init; }
+
+    /// <summary>
+    /// Reachability score in basis points.
+    /// </summary>
+    public required int ReachabilityBps { get; init; }
+
+    /// <summary>
+    /// Evidence quality score in basis points.
+    /// </summary>
+    public required int EvidenceBps { get; init; }
+
+    /// <summary>
+    /// Provenance/supply-chain score in basis points.
+    /// </summary>
+    public required int ProvenanceBps { get; init; }
+
+    /// <summary>
+    /// EPSS score in basis points (if available).
+    /// </summary>
+    public int? EpssBps { get; init; }
+
+    /// <summary>
+    /// VEX status score in basis points (if available).
+    /// </summary>
+    public int? VexBps { get; init; }
+}
+
+/// <summary>
+/// Risk tier enumeration for categorizing trust scores.
+/// </summary>
+public enum RiskTier
+{
+    Info = 0,
+    Low = 1,
+    Medium = 2,
+    High = 3,
+    Critical = 4
+}
+
+/// <summary>
+/// Request for computing a trust score.
+/// </summary>
+public sealed record TrustScoreRequest
+{
+    public required string ArtifactId { get; init; }
+    public string? VulnerabilityId { get; init; }
+    public string? TenantId { get; init; }
+    public SignalSnapshot? Signals { get; init; }
+    public ScorePolicy? PolicyOverride { get; init; }
+}
+
+/// <summary>
+/// Result from trust score computation with full explainability.
+/// </summary>
+public sealed record TrustScoreResult
+{
+    /// <summary>
+    /// The Score.v1 predicate suitable for attestation signing.
+    /// </summary>
+    public required ScoreV1Predicate Predicate { get; init; }
+
+    /// <summary>
+    /// Signal snapshot used for computation.
+    /// </summary>
+    public required SignalSnapshot SignalsUsed { get; init; }
+
+    /// <summary>
+    /// Whether the score computation succeeded.
+    /// </summary>
+    public required bool Success { get; init; }
+
+    /// <summary>
+    /// Error message if computation failed.
+    /// </summary>
+    public string? Error { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/ITriageObservationSource.cs

@@ -0,0 +1,22 @@
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Provides observations to the triage queue evaluator.
+/// Implementations may read from a database, cache, or in-memory store.
+/// </summary>
+public interface ITriageObservationSource
+{
+    /// <summary>
+    /// Retrieve observations that are candidates for triage evaluation.
+    /// The source should return observations that have not been evaluated recently
+    /// (based on <see cref="TriageQueueOptions.MinEvaluationIntervalMinutes"/>).
+    /// </summary>
+    /// <param name="tenantId">Optional tenant filter. Null returns all tenants.</param>
+    /// <param name="maxItems">Maximum number of observations to return.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Candidate observations.</returns>
+    Task<IReadOnlyList<TriageObservation>> GetCandidatesAsync(
+        string? tenantId = null,
+        int maxItems = 500,
+        CancellationToken cancellationToken = default);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/ITriageQueueEvaluator.cs

@@ -0,0 +1,27 @@
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Evaluates a batch of observations and produces a priority-sorted triage queue.
+/// </summary>
+public interface ITriageQueueEvaluator
+{
+    /// <summary>
+    /// Evaluate observations and produce a triage queue snapshot sorted by decay urgency.
+    /// </summary>
+    /// <param name="observations">Observations to evaluate.</param>
+    /// <param name="now">Reference time for decay calculation (deterministic).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Sorted triage queue snapshot.</returns>
+    Task<TriageQueueSnapshot> EvaluateAsync(
+        IReadOnlyList<TriageObservation> observations,
+        DateTimeOffset now,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Evaluate a single observation and determine if it should be queued.
+    /// </summary>
+    /// <param name="observation">The observation to evaluate.</param>
+    /// <param name="now">Reference time for decay calculation.</param>
+    /// <returns>Triage item, or null if the observation does not need triage.</returns>
+    TriageItem? EvaluateSingle(TriageObservation observation, DateTimeOffset now);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/ITriageReanalysisSink.cs

@@ -0,0 +1,18 @@
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Sink for stale observations that need re-analysis.
+/// Implementations may enqueue to an in-memory channel, message bus, or database table.
+/// </summary>
+public interface ITriageReanalysisSink
+{
+    /// <summary>
+    /// Enqueue stale observations for re-analysis.
+    /// </summary>
+    /// <param name="items">Triage items to re-analyse (already filtered to stale/approaching).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Number of items successfully enqueued.</returns>
+    Task<int> EnqueueAsync(
+        IReadOnlyList<TriageItem> items,
+        CancellationToken cancellationToken = default);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/InMemoryTriageReanalysisSink.cs

@@ -0,0 +1,94 @@
+using System.Collections.Concurrent;
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// In-memory implementation of <see cref="ITriageReanalysisSink"/>.
+/// Stores enqueued items in a thread-safe collection for consumption by re-analysis workers.
+/// Suitable for single-node deployments, testing, and offline/air-gap scenarios.
+/// </summary>
+public sealed class InMemoryTriageReanalysisSink : ITriageReanalysisSink
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization");
+
+    private static readonly Counter<long> EnqueuedCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_inmemory_enqueued_total",
+        unit: "{items}",
+        description: "Items enqueued in the in-memory triage sink");
+
+    private static readonly Counter<long> DequeuedCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_inmemory_dequeued_total",
+        unit: "{items}",
+        description: "Items dequeued from the in-memory triage sink");
+
+    private readonly ConcurrentQueue<TriageItem> _queue = new();
+    private readonly ILogger<InMemoryTriageReanalysisSink> _logger;
+
+    public InMemoryTriageReanalysisSink(ILogger<InMemoryTriageReanalysisSink> logger)
+    {
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public Task<int> EnqueueAsync(
+        IReadOnlyList<TriageItem> items,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+
+        var enqueued = 0;
+        foreach (var item in items)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            _queue.Enqueue(item);
+            enqueued++;
+        }
+
+        EnqueuedCounter.Add(enqueued);
+        _logger.LogDebug("Enqueued {Count} triage items (queue depth: {Depth})", enqueued, _queue.Count);
+
+        return Task.FromResult(enqueued);
+    }
+
+    /// <summary>
+    /// Try to dequeue the next item for re-analysis.
+    /// </summary>
+    /// <param name="item">The dequeued item, if available.</param>
+    /// <returns>True if an item was dequeued.</returns>
+    public bool TryDequeue(out TriageItem? item)
+    {
+        var result = _queue.TryDequeue(out item);
+        if (result)
+            DequeuedCounter.Add(1);
+        return result;
+    }
+
+    /// <summary>
+    /// Drain all pending items.
+    /// </summary>
+    /// <returns>All pending triage items.</returns>
+    public IReadOnlyList<TriageItem> DrainAll()
+    {
+        var items = new List<TriageItem>();
+        while (_queue.TryDequeue(out var item))
+        {
+            items.Add(item);
+        }
+
+        if (items.Count > 0)
+            DequeuedCounter.Add(items.Count);
+
+        return items;
+    }
+
+    /// <summary>
+    /// Current queue depth.
+    /// </summary>
+    public int Count => _queue.Count;
+
+    /// <summary>
+    /// Peek at all pending items without removing them.
+    /// </summary>
+    public IReadOnlyList<TriageItem> PeekAll() => _queue.ToArray();
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/TriageModels.cs

@@ -0,0 +1,172 @@
+using System.Text.Json.Serialization;
+using StellaOps.Policy.Determinization.Models;
+
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Priority classification for triage items based on decay urgency.
+/// </summary>
+public enum TriagePriority
+{
+    /// <summary>No action needed — observation is fresh.</summary>
+    None = 0,
+
+    /// <summary>Observation approaching staleness (decay multiplier 0.50–0.70).</summary>
+    Low = 1,
+
+    /// <summary>Observation is stale (decay multiplier 0.30–0.50).</summary>
+    Medium = 2,
+
+    /// <summary>Observation is heavily decayed (decay multiplier 0.10–0.30).</summary>
+    High = 3,
+
+    /// <summary>Observation at or near floor — effectively no confidence (decay multiplier ≤ 0.10).</summary>
+    Critical = 4
+}
+
+/// <summary>
+/// Represents a single unknown observation queued for triage.
+/// </summary>
+public sealed record TriageItem
+{
+    /// <summary>CVE identifier.</summary>
+    [JsonPropertyName("cve")]
+    public required string Cve { get; init; }
+
+    /// <summary>Component PURL.</summary>
+    [JsonPropertyName("purl")]
+    public required string Purl { get; init; }
+
+    /// <summary>Tenant identifier for multi-tenant isolation.</summary>
+    [JsonPropertyName("tenant_id")]
+    public required string TenantId { get; init; }
+
+    /// <summary>The observation decay state.</summary>
+    [JsonPropertyName("observation_decay")]
+    public required ObservationDecay Decay { get; init; }
+
+    /// <summary>Current decay multiplier at evaluation time.</summary>
+    [JsonPropertyName("current_multiplier")]
+    public required double CurrentMultiplier { get; init; }
+
+    /// <summary>Computed triage priority based on decay urgency.</summary>
+    [JsonPropertyName("priority")]
+    public required TriagePriority Priority { get; init; }
+
+    /// <summary>Age in days since last refresh at evaluation time.</summary>
+    [JsonPropertyName("age_days")]
+    public required double AgeDays { get; init; }
+
+    /// <summary>Days until the observation crosses the staleness threshold (negative if already stale).</summary>
+    [JsonPropertyName("days_until_stale")]
+    public required double DaysUntilStale { get; init; }
+
+    /// <summary>When this triage item was evaluated (UTC).</summary>
+    [JsonPropertyName("evaluated_at")]
+    public required DateTimeOffset EvaluatedAt { get; init; }
+
+    /// <summary>Optional signal gaps contributing to uncertainty.</summary>
+    [JsonPropertyName("signal_gaps")]
+    public IReadOnlyList<SignalGap> SignalGaps { get; init; } = [];
+
+    /// <summary>Recommended next action for the operator.</summary>
+    [JsonPropertyName("recommended_action")]
+    public string? RecommendedAction { get; init; }
+}
+
+/// <summary>
+/// Result of evaluating a batch of observations for triage.
+/// </summary>
+public sealed record TriageQueueSnapshot
+{
+    /// <summary>Items sorted by priority (Critical first) then by days-until-stale ascending.</summary>
+    [JsonPropertyName("items")]
+    public required IReadOnlyList<TriageItem> Items { get; init; }
+
+    /// <summary>Total observations evaluated.</summary>
+    [JsonPropertyName("total_evaluated")]
+    public required int TotalEvaluated { get; init; }
+
+    /// <summary>Count of items that are already stale.</summary>
+    [JsonPropertyName("stale_count")]
+    public required int StaleCount { get; init; }
+
+    /// <summary>Count of items approaching staleness (Low priority).</summary>
+    [JsonPropertyName("approaching_count")]
+    public required int ApproachingCount { get; init; }
+
+    /// <summary>When this snapshot was computed (UTC).</summary>
+    [JsonPropertyName("evaluated_at")]
+    public required DateTimeOffset EvaluatedAt { get; init; }
+
+    /// <summary>Summary statistics by priority tier.</summary>
+    [JsonPropertyName("priority_summary")]
+    public required IReadOnlyDictionary<TriagePriority, int> PrioritySummary { get; init; }
+}
+
+/// <summary>
+/// Configuration for triage queue evaluation thresholds.
+/// </summary>
+public sealed record TriageQueueOptions
+{
+    /// <summary>Default section name in appsettings.json.</summary>
+    public const string SectionName = "Determinization:TriageQueue";
+
+    /// <summary>
+    /// Multiplier threshold for "approaching staleness" (Low priority).
+    /// Observations with decay multiplier below this but above staleness are flagged.
+    /// Default: 0.70
+    /// </summary>
+    public double ApproachingThreshold { get; init; } = 0.70;
+
+    /// <summary>
+    /// Multiplier threshold for High priority.
+    /// Default: 0.30
+    /// </summary>
+    public double HighPriorityThreshold { get; init; } = 0.30;
+
+    /// <summary>
+    /// Multiplier threshold for Critical priority.
+    /// Default: 0.10
+    /// </summary>
+    public double CriticalPriorityThreshold { get; init; } = 0.10;
+
+    /// <summary>
+    /// Maximum number of items to include in a snapshot.
+    /// Default: 500
+    /// </summary>
+    public int MaxSnapshotItems { get; init; } = 500;
+
+    /// <summary>
+    /// Whether to include non-stale observations that are approaching staleness.
+    /// Default: true
+    /// </summary>
+    public bool IncludeApproaching { get; init; } = true;
+
+    /// <summary>
+    /// Minimum interval between triage evaluations for the same observation in minutes.
+    /// Default: 60
+    /// </summary>
+    public int MinEvaluationIntervalMinutes { get; init; } = 60;
+}
+
+/// <summary>
+/// Represents an observation to be evaluated for triage.
+/// </summary>
+public sealed record TriageObservation
+{
+    /// <summary>CVE identifier.</summary>
+    public required string Cve { get; init; }
+
+    /// <summary>Component PURL.</summary>
+    public required string Purl { get; init; }
+
+    /// <summary>Tenant identifier.</summary>
+    public required string TenantId { get; init; }
+
+    /// <summary>Decay state of the observation.</summary>
+    public required ObservationDecay Decay { get; init; }
+
+    /// <summary>Optional signal gaps from the most recent uncertainty evaluation.</summary>
+    public IReadOnlyList<SignalGap> SignalGaps { get; init; } = [];
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/TriageQueueEvaluator.cs

@@ -0,0 +1,227 @@
+using System.Diagnostics.Metrics;
+using StellaOps.Policy.Determinization.Models;
+
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Evaluates observations for decay-based triage and produces priority-sorted snapshots.
+/// All calculations are deterministic given the same inputs and reference time.
+/// </summary>
+public sealed class TriageQueueEvaluator : ITriageQueueEvaluator
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization");
+
+    private static readonly Counter<long> ItemsEvaluatedCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_items_evaluated_total",
+        unit: "{items}",
+        description: "Total observations evaluated for triage");
+
+    private static readonly Counter<long> ItemsQueuedCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_items_queued_total",
+        unit: "{items}",
+        description: "Observations added to triage queue");
+
+    private static readonly Histogram<double> DecayMultiplierHistogram = Meter.CreateHistogram<double>(
+        "stellaops_triage_decay_multiplier",
+        unit: "ratio",
+        description: "Decay multiplier distribution of triage items");
+
+    private readonly ILogger<TriageQueueEvaluator> _logger;
+    private readonly TriageQueueOptions _options;
+
+    public TriageQueueEvaluator(
+        ILogger<TriageQueueEvaluator> logger,
+        IOptions<TriageQueueOptions> options)
+    {
+        _logger = logger;
+        _options = options.Value;
+    }
+
+    /// <inheritdoc />
+    public Task<TriageQueueSnapshot> EvaluateAsync(
+        IReadOnlyList<TriageObservation> observations,
+        DateTimeOffset now,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(observations);
+
+        var triageItems = new List<TriageItem>();
+
+        foreach (var obs in observations)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            var item = EvaluateSingle(obs, now);
+            if (item is not null)
+            {
+                triageItems.Add(item);
+            }
+        }
+
+        // Sort: Critical first, then by days-until-stale ascending (most urgent first)
+        var sorted = triageItems
+            .OrderByDescending(i => i.Priority)
+            .ThenBy(i => i.DaysUntilStale)
+            .ThenBy(i => i.Cve, StringComparer.Ordinal)
+            .ThenBy(i => i.Purl, StringComparer.Ordinal)
+            .Take(_options.MaxSnapshotItems)
+            .ToList();
+
+        // Compute summary
+        var prioritySummary = new Dictionary<TriagePriority, int>();
+        foreach (var priority in Enum.GetValues<TriagePriority>())
+        {
+            var count = sorted.Count(i => i.Priority == priority);
+            if (count > 0)
+                prioritySummary[priority] = count;
+        }
+
+        var staleCount = sorted.Count(i => i.DaysUntilStale < 0);
+        var approachingCount = sorted.Count(i => i.Priority == TriagePriority.Low);
+
+        var snapshot = new TriageQueueSnapshot
+        {
+            Items = sorted,
+            TotalEvaluated = observations.Count,
+            StaleCount = staleCount,
+            ApproachingCount = approachingCount,
+            EvaluatedAt = now,
+            PrioritySummary = prioritySummary
+        };
+
+        // Emit metrics
+        ItemsEvaluatedCounter.Add(observations.Count);
+        ItemsQueuedCounter.Add(sorted.Count);
+
+        _logger.LogInformation(
+            "Triage evaluation: {Total} observations, {Queued} queued ({Stale} stale, {Approaching} approaching)",
+            observations.Count,
+            sorted.Count,
+            staleCount,
+            approachingCount);
+
+        return Task.FromResult(snapshot);
+    }
+
+    /// <inheritdoc />
+    public TriageItem? EvaluateSingle(TriageObservation observation, DateTimeOffset now)
+    {
+        ArgumentNullException.ThrowIfNull(observation);
+
+        var decay = observation.Decay;
+        var multiplier = decay.CalculateDecay(now);
+        var ageDays = (now - decay.RefreshedAt).TotalDays;
+        var isStale = decay.CheckIsStale(now);
+        var priority = ClassifyPriority(multiplier, decay.StalenessThreshold);
+
+        // Skip if not stale and not approaching (unless IncludeApproaching is true)
+        if (priority == TriagePriority.None)
+            return null;
+
+        if (priority == TriagePriority.Low && !_options.IncludeApproaching)
+            return null;
+
+        var daysUntilStale = CalculateDaysUntilStale(
+            decay.RefreshedAt,
+            decay.HalfLifeDays,
+            decay.StalenessThreshold,
+            decay.Floor,
+            now);
+
+        var recommendedAction = DetermineRecommendedAction(priority, observation.SignalGaps);
+
+        // Emit per-item metric
+        DecayMultiplierHistogram.Record(multiplier,
+            new KeyValuePair<string, object?>("priority", priority.ToString()),
+            new KeyValuePair<string, object?>("tenant_id", observation.TenantId));
+
+        return new TriageItem
+        {
+            Cve = observation.Cve,
+            Purl = observation.Purl,
+            TenantId = observation.TenantId,
+            Decay = decay,
+            CurrentMultiplier = multiplier,
+            Priority = priority,
+            AgeDays = Math.Max(0.0, ageDays),
+            DaysUntilStale = daysUntilStale,
+            EvaluatedAt = now,
+            SignalGaps = observation.SignalGaps,
+            RecommendedAction = recommendedAction
+        };
+    }
+
+    /// <summary>
+    /// Classifies triage priority based on current decay multiplier.
+    /// </summary>
+    internal TriagePriority ClassifyPriority(double multiplier, double stalenessThreshold)
+    {
+        if (multiplier <= _options.CriticalPriorityThreshold)
+            return TriagePriority.Critical;
+
+        if (multiplier <= _options.HighPriorityThreshold)
+            return TriagePriority.High;
+
+        if (multiplier <= stalenessThreshold)
+            return TriagePriority.Medium;
+
+        if (multiplier <= _options.ApproachingThreshold)
+            return TriagePriority.Low;
+
+        return TriagePriority.None;
+    }
+
+    /// <summary>
+    /// Calculates days until the observation crosses the staleness threshold.
+    /// Negative values indicate the observation is already stale.
+    /// Formula: days = -halfLife * ln(threshold) / ln(2), solving exp(-ln(2) * days / halfLife) = threshold
+    /// </summary>
+    internal static double CalculateDaysUntilStale(
+        DateTimeOffset refreshedAt,
+        double halfLifeDays,
+        double stalenessThreshold,
+        double floor,
+        DateTimeOffset now)
+    {
+        // If floor >= threshold, the observation can never become stale via decay alone
+        if (floor >= stalenessThreshold)
+            return double.MaxValue;
+
+        // Days at which multiplier crosses threshold:
+        // threshold = exp(-ln(2) * d / halfLife)
+        // ln(threshold) = -ln(2) * d / halfLife
+        // d = -halfLife * ln(threshold) / ln(2)
+        var daysToThreshold = -halfLifeDays * Math.Log(stalenessThreshold) / Math.Log(2.0);
+        var currentAgeDays = (now - refreshedAt).TotalDays;
+
+        return daysToThreshold - currentAgeDays;
+    }
+
+    /// <summary>
+    /// Determines a recommended action based on priority and signal gaps.
+    /// </summary>
+    private static string? DetermineRecommendedAction(TriagePriority priority, IReadOnlyList<SignalGap> gaps)
+    {
+        if (gaps.Count > 0)
+        {
+            var missingSignals = string.Join(", ", gaps.Select(g => g.Signal));
+            return priority switch
+            {
+                TriagePriority.Critical => $"URGENT: Re-analyse immediately. Missing signals: {missingSignals}",
+                TriagePriority.High => $"Re-analyse soon. Missing signals: {missingSignals}",
+                TriagePriority.Medium => $"Schedule re-analysis. Missing signals: {missingSignals}",
+                TriagePriority.Low => $"Monitor — approaching staleness. Missing signals: {missingSignals}",
+                _ => null
+            };
+        }
+
+        return priority switch
+        {
+            TriagePriority.Critical => "URGENT: Re-analyse immediately — evidence has decayed to floor",
+            TriagePriority.High => "Re-analyse soon — evidence is heavily decayed",
+            TriagePriority.Medium => "Schedule re-analysis — observation is stale",
+            TriagePriority.Low => "Monitor — observation is approaching staleness",
+            _ => null
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/Triage/UnknownTriageQueueService.cs

@@ -0,0 +1,139 @@
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Policy.Determinization.Scoring.Triage;
+
+/// <summary>
+/// Background service that periodically evaluates observations for decay-based staleness
+/// and enqueues stale unknowns for re-analysis.
+///
+/// This service is the event-driven mechanism that bridges ObservationDecay.CheckIsStale()
+/// with the re-analysis pipeline, fulfilling the automated re-analysis triggering requirement.
+/// </summary>
+public sealed class UnknownTriageQueueService
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization");
+
+    private static readonly Counter<long> CyclesCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_cycles_total",
+        unit: "{cycles}",
+        description: "Total triage evaluation cycles executed");
+
+    private static readonly Counter<long> EnqueuedCounter = Meter.CreateCounter<long>(
+        "stellaops_triage_reanalysis_enqueued_total",
+        unit: "{items}",
+        description: "Total items enqueued for re-analysis");
+
+    private static readonly Histogram<double> CycleDurationHistogram = Meter.CreateHistogram<double>(
+        "stellaops_triage_cycle_duration_seconds",
+        unit: "s",
+        description: "Duration of triage evaluation cycles");
+
+    private readonly ITriageQueueEvaluator _evaluator;
+    private readonly ITriageObservationSource _source;
+    private readonly ITriageReanalysisSink _sink;
+    private readonly ILogger<UnknownTriageQueueService> _logger;
+    private readonly TriageQueueOptions _options;
+    private readonly TimeProvider _timeProvider;
+
+    public UnknownTriageQueueService(
+        ITriageQueueEvaluator evaluator,
+        ITriageObservationSource source,
+        ITriageReanalysisSink sink,
+        ILogger<UnknownTriageQueueService> logger,
+        IOptions<TriageQueueOptions> options,
+        TimeProvider? timeProvider = null)
+    {
+        _evaluator = evaluator;
+        _source = source;
+        _sink = sink;
+        _logger = logger;
+        _options = options.Value;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+    }
+
+    /// <summary>
+    /// Execute a single triage cycle: fetch candidates, evaluate, enqueue stale items.
+    /// This method is designed to be called by a background host, timer, or scheduler.
+    /// </summary>
+    /// <param name="tenantId">Optional tenant filter.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The triage snapshot from this cycle.</returns>
+    public async Task<TriageQueueSnapshot> ExecuteCycleAsync(
+        string? tenantId = null,
+        CancellationToken cancellationToken = default)
+    {
+        var now = _timeProvider.GetUtcNow();
+        var sw = System.Diagnostics.Stopwatch.StartNew();
+
+        _logger.LogInformation(
+            "Starting triage cycle at {Now:O} for tenant {TenantId}",
+            now,
+            tenantId ?? "(all)");
+
+        try
+        {
+            // 1. Fetch candidate observations
+            var candidates = await _source.GetCandidatesAsync(
+                tenantId,
+                _options.MaxSnapshotItems,
+                cancellationToken);
+
+            _logger.LogDebug("Fetched {Count} candidate observations", candidates.Count);
+
+            // 2. Evaluate for triage
+            var snapshot = await _evaluator.EvaluateAsync(candidates, now, cancellationToken);
+
+            // 3. Enqueue stale items for re-analysis (Medium, High, Critical)
+            var reanalysisItems = snapshot.Items
+                .Where(i => i.Priority >= TriagePriority.Medium)
+                .ToList();
+
+            var enqueued = 0;
+            if (reanalysisItems.Count > 0)
+            {
+                enqueued = await _sink.EnqueueAsync(reanalysisItems, cancellationToken);
+                EnqueuedCounter.Add(enqueued);
+            }
+
+            sw.Stop();
+            CyclesCounter.Add(1);
+            CycleDurationHistogram.Record(sw.Elapsed.TotalSeconds,
+                new KeyValuePair<string, object?>("tenant_id", tenantId ?? "all"));
+
+            _logger.LogInformation(
+                "Triage cycle complete: {Evaluated} evaluated, {Queued} queued, {Enqueued} enqueued for re-analysis ({Duration:F2}s)",
+                snapshot.TotalEvaluated,
+                snapshot.Items.Count,
+                enqueued,
+                sw.Elapsed.TotalSeconds);
+
+            return snapshot;
+        }
+        catch (OperationCanceledException)
+        {
+            _logger.LogWarning("Triage cycle cancelled");
+            throw;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Triage cycle failed");
+            throw;
+        }
+    }
+
+    /// <summary>
+    /// Evaluate a specific set of observations (for on-demand triage, e.g. CLI/API).
+    /// Does not enqueue — returns the snapshot for the caller to act on.
+    /// </summary>
+    /// <param name="observations">Observations to evaluate.</param>
+    /// <param name="now">Reference time.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Triage queue snapshot.</returns>
+    public Task<TriageQueueSnapshot> EvaluateOnDemandAsync(
+        IReadOnlyList<TriageObservation> observations,
+        DateTimeOffset now,
+        CancellationToken cancellationToken = default)
+    {
+        return _evaluator.EvaluateAsync(observations, now, cancellationToken);
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/TrustScoreAlgebraFacade.cs

@@ -0,0 +1,340 @@
+using Microsoft.Extensions.Logging;
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Scoring;
+using StellaOps.Policy.TrustLattice;
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+
+namespace StellaOps.Policy.Determinization.Scoring;
+
+/// <summary>
+/// Unified facade composing TrustScoreAggregator + K4Lattice + ScorePolicy into a single
+/// deterministic scoring pipeline. Entry point for computing trust scores with full
+/// explainability and attestation-ready output.
+/// </summary>
+public interface ITrustScoreAlgebraFacade
+{
+    /// <summary>
+    /// Compute a complete trust score for an artifact.
+    /// </summary>
+    /// <param name="request">Scoring request with artifact, signals, and optional policy override.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Complete scoring result with Score.v1 predicate.</returns>
+    Task<TrustScoreResult> ComputeTrustScoreAsync(
+        TrustScoreRequest request,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Compute trust score synchronously (for batch/offline use).
+    /// </summary>
+    TrustScoreResult ComputeTrustScore(TrustScoreRequest request);
+}
+
+/// <summary>
+/// Implementation of the trust score algebra facade.
+/// Composes all scoring components into a deterministic pipeline.
+/// </summary>
+public sealed class TrustScoreAlgebraFacade : ITrustScoreAlgebraFacade
+{
+    private readonly TrustScoreAggregator _aggregator;
+    private readonly UncertaintyScoreCalculator _uncertaintyCalculator;
+    private readonly ILogger<TrustScoreAlgebraFacade> _logger;
+    private readonly TimeProvider _timeProvider;
+    private readonly JsonSerializerOptions _jsonOptions;
+
+    public TrustScoreAlgebraFacade(
+        TrustScoreAggregator aggregator,
+        UncertaintyScoreCalculator uncertaintyCalculator,
+        ILogger<TrustScoreAlgebraFacade>? logger = null,
+        TimeProvider? timeProvider = null)
+    {
+        _aggregator = aggregator ?? throw new ArgumentNullException(nameof(aggregator));
+        _uncertaintyCalculator = uncertaintyCalculator ?? throw new ArgumentNullException(nameof(uncertaintyCalculator));
+        _logger = logger ?? Microsoft.Extensions.Logging.Abstractions.NullLogger<TrustScoreAlgebraFacade>.Instance;
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _jsonOptions = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+            WriteIndented = false
+        };
+    }
+
+    /// <inheritdoc />
+    public Task<TrustScoreResult> ComputeTrustScoreAsync(
+        TrustScoreRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        // Scoring is CPU-bound and deterministic; run synchronously
+        var result = ComputeTrustScore(request);
+        return Task.FromResult(result);
+    }
+
+    /// <inheritdoc />
+    public TrustScoreResult ComputeTrustScore(TrustScoreRequest request)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+        ArgumentException.ThrowIfNullOrWhiteSpace(request.ArtifactId);
+
+        try
+        {
+            var now = _timeProvider.GetUtcNow();
+            var policy = request.PolicyOverride ?? ScorePolicy.Default;
+            var signals = request.Signals ?? SignalSnapshot.Empty(
+                request.VulnerabilityId ?? "UNKNOWN",
+                request.ArtifactId,
+                now);
+
+            // Step 1: Calculate uncertainty score
+            var uncertaintyScore = _uncertaintyCalculator.Calculate(signals);
+
+            // Step 2: Aggregate signals using weighted formula
+            var trustScore = _aggregator.Aggregate(signals, uncertaintyScore);
+
+            // Step 3: Compute K4 lattice verdict
+            var latticeVerdict = ComputeLatticeVerdict(signals);
+
+            // Step 4: Extract dimension scores
+            var dimensions = ExtractDimensions(signals, policy);
+
+            // Step 5: Compute weighted final score in basis points
+            var finalBps = ComputeWeightedScoreBps(dimensions, policy.WeightsBps);
+
+            // Step 6: Determine risk tier
+            var tier = DetermineRiskTier(finalBps);
+
+            // Step 7: Compute policy digest
+            var policyDigest = ComputePolicyDigest(policy);
+
+            // Step 8: Build Score.v1 predicate
+            var predicate = new ScoreV1Predicate
+            {
+                ArtifactId = request.ArtifactId,
+                VulnerabilityId = request.VulnerabilityId,
+                TrustScoreBps = finalBps,
+                Tier = tier.ToString(),
+                LatticeVerdict = latticeVerdict,
+                UncertaintyBps = ToBasisPoints(uncertaintyScore.Entropy),
+                Dimensions = dimensions,
+                WeightsUsed = policy.WeightsBps,
+                PolicyDigest = policyDigest,
+                ComputedAt = now,
+                TenantId = request.TenantId
+            };
+
+            _logger.LogDebug(
+                "Computed trust score for {ArtifactId}: {ScoreBps}bps ({Tier}), lattice={Verdict}",
+                request.ArtifactId, finalBps, tier, latticeVerdict);
+
+            return new TrustScoreResult
+            {
+                Predicate = predicate,
+                SignalsUsed = signals,
+                Success = true
+            };
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Failed to compute trust score for {ArtifactId}", request.ArtifactId);
+            return new TrustScoreResult
+            {
+                Predicate = CreateEmptyPredicate(request),
+                SignalsUsed = request.Signals ?? SignalSnapshot.Empty(
+                    request.VulnerabilityId ?? "UNKNOWN",
+                    request.ArtifactId,
+                    _timeProvider.GetUtcNow()),
+                Success = false,
+                Error = ex.Message
+            };
+        }
+    }
+
+    /// <summary>
+    /// Compute K4 lattice verdict from signal states.
+    /// </summary>
+    private static K4Value ComputeLatticeVerdict(SignalSnapshot signals)
+    {
+        var values = new List<K4Value>();
+
+        // Map each signal to K4 value
+        if (!signals.Vex.IsNotQueried)
+        {
+            values.Add(signals.Vex.Value?.Status?.ToLowerInvariant() switch
+            {
+                "affected" => K4Value.True,           // Vulnerability confirmed
+                "not_affected" => K4Value.False,      // Vulnerability not present
+                "fixed" => K4Value.False,             // Fixed = not vulnerable
+                "under_investigation" => K4Value.Unknown,
+                _ => K4Value.Unknown
+            });
+        }
+
+        if (!signals.Reachability.IsNotQueried)
+        {
+            values.Add(signals.Reachability.Value?.Status switch
+            {
+                ReachabilityStatus.Reachable => K4Value.True,
+                ReachabilityStatus.Unreachable => K4Value.False,
+                ReachabilityStatus.Unknown => K4Value.Unknown,
+                _ => K4Value.Unknown
+            });
+        }
+
+        if (!signals.Epss.IsNotQueried && signals.Epss.Value is not null)
+        {
+            // High EPSS = likely exploitable
+            values.Add(signals.Epss.Value.Epss >= 0.5 ? K4Value.True : K4Value.False);
+        }
+
+        // Join all values using K4 lattice
+        return K4Lattice.JoinAll(values);
+    }
+
+    /// <summary>
+    /// Extract dimension scores from signals.
+    /// </summary>
+    private static ScoreDimensionsBps ExtractDimensions(SignalSnapshot signals, ScorePolicy policy)
+    {
+        // Base severity from CVSS or default
+        var baseSeverityBps = 5000; // Default to medium if no CVSS
+
+        // Reachability
+        var reachabilityBps = signals.Reachability.Value?.Status switch
+        {
+            ReachabilityStatus.Reachable => 10000,
+            ReachabilityStatus.Unreachable => 0,
+            _ => 5000 // Unknown = mid-range
+        };
+
+        // Evidence quality (based on how many signals are present)
+        var signalCount = CountPresentSignals(signals);
+        var evidenceBps = signalCount switch
+        {
+            >= 5 => 9000,
+            4 => 7500,
+            3 => 6000,
+            2 => 4000,
+            1 => 2000,
+            _ => 1000
+        };
+
+        // Provenance (SBOM lineage quality)
+        var provenanceBps = signals.Sbom.Value is not null ? 8000 : 3000;
+
+        // Optional dimensions
+        int? epssBps = signals.Epss.Value is not null
+            ? ToBasisPoints(signals.Epss.Value.Epss)
+            : null;
+
+        int? vexBps = signals.Vex.Value?.Status?.ToLowerInvariant() switch
+        {
+            "affected" => 10000,
+            "under_investigation" => 7000,
+            "fixed" => 1000,
+            "not_affected" => 0,
+            _ => null
+        };
+
+        return new ScoreDimensionsBps
+        {
+            BaseSeverityBps = baseSeverityBps,
+            ReachabilityBps = reachabilityBps,
+            EvidenceBps = evidenceBps,
+            ProvenanceBps = provenanceBps,
+            EpssBps = epssBps,
+            VexBps = vexBps
+        };
+    }
+
+    /// <summary>
+    /// Compute final weighted score in basis points.
+    /// </summary>
+    private static int ComputeWeightedScoreBps(ScoreDimensionsBps dimensions, WeightsBps weights)
+    {
+        // Weighted average: Σ(dimension * weight) / Σ(weights)
+        // Since weights sum to 10000, we can use: Σ(dimension * weight) / 10000
+
+        long weighted =
+            (long)dimensions.BaseSeverityBps * weights.BaseSeverity +
+            (long)dimensions.ReachabilityBps * weights.Reachability +
+            (long)dimensions.EvidenceBps * weights.Evidence +
+            (long)dimensions.ProvenanceBps * weights.Provenance;
+
+        var result = (int)(weighted / 10000);
+        return Math.Clamp(result, 0, 10000);
+    }
+
+    /// <summary>
+    /// Determine risk tier from basis point score.
+    /// </summary>
+    private static RiskTier DetermineRiskTier(int scoreBps)
+    {
+        return scoreBps switch
+        {
+            >= 9000 => RiskTier.Critical,
+            >= 7000 => RiskTier.High,
+            >= 4000 => RiskTier.Medium,
+            >= 1000 => RiskTier.Low,
+            _ => RiskTier.Info
+        };
+    }
+
+    /// <summary>
+    /// Compute SHA-256 digest of policy for reproducibility.
+    /// </summary>
+    private string ComputePolicyDigest(ScorePolicy policy)
+    {
+        var json = JsonSerializer.Serialize(policy, _jsonOptions);
+        var bytes = SHA256.HashData(Encoding.UTF8.GetBytes(json));
+        return Convert.ToHexStringLower(bytes);
+    }
+
+    /// <summary>
+    /// Count present (non-null, non-queried) signals.
+    /// </summary>
+    private static int CountPresentSignals(SignalSnapshot signals)
+    {
+        var count = 0;
+        if (!signals.Vex.IsNotQueried && signals.Vex.Value is not null) count++;
+        if (!signals.Epss.IsNotQueried && signals.Epss.Value is not null) count++;
+        if (!signals.Reachability.IsNotQueried && signals.Reachability.Value is not null) count++;
+        if (!signals.Runtime.IsNotQueried && signals.Runtime.Value is not null) count++;
+        if (!signals.Backport.IsNotQueried && signals.Backport.Value is not null) count++;
+        if (!signals.Sbom.IsNotQueried && signals.Sbom.Value is not null) count++;
+        return count;
+    }
+
+    /// <summary>
+    /// Convert a 0.0-1.0 double to basis points.
+    /// </summary>
+    private static int ToBasisPoints(double value) =>
+        Math.Clamp((int)(value * 10000), 0, 10000);
+
+    /// <summary>
+    /// Create empty predicate for error cases.
+    /// </summary>
+    private ScoreV1Predicate CreateEmptyPredicate(TrustScoreRequest request)
+    {
+        return new ScoreV1Predicate
+        {
+            ArtifactId = request.ArtifactId,
+            VulnerabilityId = request.VulnerabilityId,
+            TrustScoreBps = 0,
+            Tier = RiskTier.Info.ToString(),
+            LatticeVerdict = K4Value.Unknown,
+            UncertaintyBps = 10000,
+            Dimensions = new ScoreDimensionsBps
+            {
+                BaseSeverityBps = 0,
+                ReachabilityBps = 0,
+                EvidenceBps = 0,
+                ProvenanceBps = 0
+            },
+            WeightsUsed = WeightsBps.Default,
+            PolicyDigest = "error",
+            ComputedAt = _timeProvider.GetUtcNow(),
+            TenantId = request.TenantId
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/WeightManifest/IWeightManifestLoader.cs

@@ -0,0 +1,59 @@
+// -----------------------------------------------------------------------------
+// IWeightManifestLoader.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Weight manifest loader interface
+// Description: Contract for discovering, loading, validating, and selecting
+//              versioned weight manifests from the file system.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+
+namespace StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+/// <summary>
+/// Discovers, loads, validates, and selects versioned weight manifests.
+/// </summary>
+public interface IWeightManifestLoader
+{
+    /// <summary>
+    /// Lists all available weight manifests discovered in the configured directory,
+    /// sorted by <c>effectiveFrom</c> descending (most recent first).
+    /// </summary>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>All discovered manifest load results.</returns>
+    Task<ImmutableArray<WeightManifestLoadResult>> ListAsync(CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Loads and validates a specific manifest file by path.
+    /// </summary>
+    /// <param name="filePath">Absolute or relative path to the manifest file.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Load result containing the manifest and hash verification status.</returns>
+    Task<WeightManifestLoadResult> LoadAsync(string filePath, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Selects the manifest effective for a given reference date.
+    /// Picks the most recent manifest where <c>effectiveFrom ≤ referenceDate</c>.
+    /// </summary>
+    /// <param name="referenceDate">The date to select for (typically <c>DateTimeOffset.UtcNow</c>).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The applicable manifest, or <c>null</c> if none is effective.</returns>
+    Task<WeightManifestLoadResult?> SelectEffectiveAsync(
+        DateTimeOffset referenceDate,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Validates a manifest: schema version, weight normalization, content hash.
+    /// </summary>
+    /// <param name="result">The load result to validate.</param>
+    /// <returns>Validation issues found (empty if valid).</returns>
+    ImmutableArray<string> Validate(WeightManifestLoadResult result);
+
+    /// <summary>
+    /// Computes a diff between two manifests, comparing weight values and thresholds.
+    /// </summary>
+    /// <param name="from">Source (older) manifest.</param>
+    /// <param name="to">Target (newer) manifest.</param>
+    /// <returns>Diff summary.</returns>
+    WeightManifestDiff Diff(WeightManifestDocument from, WeightManifestDocument to);
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/WeightManifest/WeightManifestCommands.cs

@@ -0,0 +1,277 @@
+// -----------------------------------------------------------------------------
+// WeightManifestCommands.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - CLI weight management commands
+// Description: Service-level commands that back the `stella weights` CLI:
+//              list, validate, diff, activate, hash. Each produces a
+//              deterministic, serializable result model.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Text.Json;
+
+namespace StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+/// <summary>
+/// Provides the backing logic for CLI weight management commands:
+/// <c>stella weights list</c>, <c>stella weights validate</c>,
+/// <c>stella weights diff</c>, <c>stella weights activate</c>,
+/// <c>stella weights hash</c>.
+/// </summary>
+public sealed class WeightManifestCommands
+{
+    private readonly IWeightManifestLoader _loader;
+
+    public WeightManifestCommands(IWeightManifestLoader loader)
+    {
+        _loader = loader ?? throw new ArgumentNullException(nameof(loader));
+    }
+
+    // ── stella weights list ──────────────────────────────────────────────
+
+    /// <summary>
+    /// Lists all discovered weight manifests with their versions,
+    /// effective dates, profiles, and hash status.
+    /// </summary>
+    public async Task<WeightsListResult> ListAsync(CancellationToken ct = default)
+    {
+        var manifests = await _loader.ListAsync(ct).ConfigureAwait(false);
+
+        var entries = manifests.Select(r => new WeightsListEntry
+        {
+            Version = r.Manifest.Version,
+            EffectiveFrom = r.Manifest.EffectiveFrom,
+            Profile = r.Manifest.Profile,
+            HashStatus = r.Manifest.HasComputedHash
+                ? (r.HashVerified ? "verified" : "mismatch")
+                : "auto",
+            SourcePath = r.SourcePath,
+            Description = r.Manifest.Description
+        }).ToImmutableArray();
+
+        return new WeightsListResult { Entries = entries };
+    }
+
+    // ── stella weights validate ──────────────────────────────────────────
+
+    /// <summary>
+    /// Validates a specific manifest file or all discovered manifests.
+    /// </summary>
+    /// <param name="filePath">
+    /// If specified, validate only this file. Otherwise validate all discovered manifests.
+    /// </param>
+    public async Task<WeightsValidateResult> ValidateAsync(
+        string? filePath = null,
+        CancellationToken ct = default)
+    {
+        var results = new List<WeightsValidateEntry>();
+
+        if (!string.IsNullOrEmpty(filePath))
+        {
+            var loadResult = await _loader.LoadAsync(filePath, ct).ConfigureAwait(false);
+            var issues = _loader.Validate(loadResult);
+            results.Add(new WeightsValidateEntry
+            {
+                Version = loadResult.Manifest.Version,
+                SourcePath = loadResult.SourcePath,
+                Issues = issues,
+                IsValid = issues.IsEmpty
+            });
+        }
+        else
+        {
+            var all = await _loader.ListAsync(ct).ConfigureAwait(false);
+            foreach (var loadResult in all)
+            {
+                var issues = _loader.Validate(loadResult);
+                results.Add(new WeightsValidateEntry
+                {
+                    Version = loadResult.Manifest.Version,
+                    SourcePath = loadResult.SourcePath,
+                    Issues = issues,
+                    IsValid = issues.IsEmpty
+                });
+            }
+        }
+
+        return new WeightsValidateResult
+        {
+            Entries = [.. results],
+            AllValid = results.TrueForAll(e => e.IsValid)
+        };
+    }
+
+    // ── stella weights diff ──────────────────────────────────────────────
+
+    /// <summary>
+    /// Diffs two manifest files, or two versions by version identifier.
+    /// </summary>
+    public async Task<WeightManifestDiff> DiffAsync(
+        string fromPath,
+        string toPath,
+        CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(fromPath);
+        ArgumentException.ThrowIfNullOrWhiteSpace(toPath);
+
+        var from = await _loader.LoadAsync(fromPath, ct).ConfigureAwait(false);
+        var to = await _loader.LoadAsync(toPath, ct).ConfigureAwait(false);
+
+        return _loader.Diff(from.Manifest, to.Manifest);
+    }
+
+    /// <summary>
+    /// Diffs two manifests by version string (searches the discovered set).
+    /// </summary>
+    public async Task<WeightManifestDiff> DiffByVersionAsync(
+        string fromVersion,
+        string toVersion,
+        CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(fromVersion);
+        ArgumentException.ThrowIfNullOrWhiteSpace(toVersion);
+
+        var all = await _loader.ListAsync(ct).ConfigureAwait(false);
+
+        var from = all.FirstOrDefault(r =>
+            string.Equals(r.Manifest.Version, fromVersion, StringComparison.OrdinalIgnoreCase));
+        var to = all.FirstOrDefault(r =>
+            string.Equals(r.Manifest.Version, toVersion, StringComparison.OrdinalIgnoreCase));
+
+        if (from is null)
+            throw new WeightManifestLoadException($"Manifest version '{fromVersion}' not found.");
+        if (to is null)
+            throw new WeightManifestLoadException($"Manifest version '{toVersion}' not found.");
+
+        return _loader.Diff(from.Manifest, to.Manifest);
+    }
+
+    // ── stella weights activate ──────────────────────────────────────────
+
+    /// <summary>
+    /// Selects the currently active (effective) manifest for a given date.
+    /// </summary>
+    public async Task<WeightsActivateResult> ActivateAsync(
+        DateTimeOffset? referenceDate = null,
+        CancellationToken ct = default)
+    {
+        var date = referenceDate ?? DateTimeOffset.UtcNow;
+        var result = await _loader.SelectEffectiveAsync(date, ct).ConfigureAwait(false);
+
+        if (result is null)
+        {
+            return new WeightsActivateResult
+            {
+                Found = false,
+                ReferenceDate = date,
+                Version = null,
+                SourcePath = null,
+                ContentHash = null
+            };
+        }
+
+        return new WeightsActivateResult
+        {
+            Found = true,
+            ReferenceDate = date,
+            Version = result.Manifest.Version,
+            SourcePath = result.SourcePath,
+            ContentHash = result.ComputedHash,
+            EffectiveFrom = result.Manifest.EffectiveFrom,
+            Profile = result.Manifest.Profile
+        };
+    }
+
+    // ── stella weights hash ──────────────────────────────────────────────
+
+    /// <summary>
+    /// Computes the content hash for a manifest file and optionally replaces
+    /// the "sha256:auto" placeholder in-place.
+    /// </summary>
+    /// <param name="filePath">Path to the manifest file.</param>
+    /// <param name="writeBack">If true, writes the computed hash back to the file.</param>
+    public async Task<WeightsHashResult> HashAsync(
+        string filePath,
+        bool writeBack = false,
+        CancellationToken ct = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(filePath);
+
+        var resolvedPath = Path.GetFullPath(filePath);
+        var json = await File.ReadAllTextAsync(resolvedPath, ct).ConfigureAwait(false);
+
+        var computedHash = WeightManifestHashComputer.ComputeFromJson(json);
+        var hasPlaceholder = json.Contains(
+            WeightManifestConstants.AutoHashPlaceholder, StringComparison.Ordinal);
+
+        string? updatedJson = null;
+        if (writeBack && hasPlaceholder)
+        {
+            var (updated, _) = WeightManifestHashComputer.ReplaceAutoHash(json);
+            updatedJson = updated;
+            await File.WriteAllTextAsync(resolvedPath, updatedJson, ct).ConfigureAwait(false);
+        }
+
+        return new WeightsHashResult
+        {
+            SourcePath = resolvedPath,
+            ComputedHash = computedHash,
+            HadPlaceholder = hasPlaceholder,
+            WrittenBack = writeBack && hasPlaceholder
+        };
+    }
+}
+
+// ── CLI result models ────────────────────────────────────────────────────────
+
+/// <summary>Result of <c>stella weights list</c>.</summary>
+public sealed record WeightsListResult
+{
+    public required ImmutableArray<WeightsListEntry> Entries { get; init; }
+}
+
+public sealed record WeightsListEntry
+{
+    public required string Version { get; init; }
+    public required DateTimeOffset EffectiveFrom { get; init; }
+    public required string Profile { get; init; }
+    public required string HashStatus { get; init; }
+    public required string SourcePath { get; init; }
+    public string? Description { get; init; }
+}
+
+/// <summary>Result of <c>stella weights validate</c>.</summary>
+public sealed record WeightsValidateResult
+{
+    public required ImmutableArray<WeightsValidateEntry> Entries { get; init; }
+    public required bool AllValid { get; init; }
+}
+
+public sealed record WeightsValidateEntry
+{
+    public required string Version { get; init; }
+    public required string SourcePath { get; init; }
+    public required ImmutableArray<string> Issues { get; init; }
+    public required bool IsValid { get; init; }
+}
+
+/// <summary>Result of <c>stella weights activate</c>.</summary>
+public sealed record WeightsActivateResult
+{
+    public required bool Found { get; init; }
+    public required DateTimeOffset ReferenceDate { get; init; }
+    public string? Version { get; init; }
+    public string? SourcePath { get; init; }
+    public string? ContentHash { get; init; }
+    public DateTimeOffset? EffectiveFrom { get; init; }
+    public string? Profile { get; init; }
+}
+
+/// <summary>Result of <c>stella weights hash</c>.</summary>
+public sealed record WeightsHashResult
+{
+    public required string SourcePath { get; init; }
+    public required string ComputedHash { get; init; }
+    public required bool HadPlaceholder { get; init; }
+    public required bool WrittenBack { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/WeightManifest/WeightManifestHashComputer.cs

@@ -0,0 +1,185 @@
+// -----------------------------------------------------------------------------
+// WeightManifestHashComputer.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Content hash auto-compute
+// Description: Deterministic SHA-256 content hash computation for weight
+//              manifests. Hashes the canonical content (excluding the
+//              contentHash field itself) to produce a stable digest.
+// -----------------------------------------------------------------------------
+
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+
+namespace StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+/// <summary>
+/// Computes deterministic SHA-256 content hashes for weight manifests.
+/// The hash covers all content except the <c>contentHash</c> field itself.
+/// </summary>
+public static class WeightManifestHashComputer
+{
+    private static readonly JsonSerializerOptions CanonicalOptions = new()
+    {
+        WriteIndented = false,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull
+    };
+
+    /// <summary>
+    /// Computes the SHA-256 content hash for a manifest's raw JSON content.
+    /// The <c>contentHash</c> field is excluded from the hash input to allow
+    /// the hash to be embedded in the same document it covers.
+    /// </summary>
+    /// <param name="jsonContent">Raw JSON content of the manifest file.</param>
+    /// <returns>Hash in "sha256:&lt;hex&gt;" format.</returns>
+    public static string ComputeFromJson(string jsonContent)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(jsonContent);
+
+        // Parse JSON, remove contentHash field, re-serialize canonically
+        var doc = JsonDocument.Parse(jsonContent);
+        var canonical = BuildCanonicalContent(doc.RootElement);
+
+        var hashBytes = SHA256.HashData(Encoding.UTF8.GetBytes(canonical));
+        return $"{WeightManifestConstants.HashPrefix}{Convert.ToHexStringLower(hashBytes)}";
+    }
+
+    /// <summary>
+    /// Computes the SHA-256 content hash for a deserialized manifest.
+    /// Re-serializes with the <c>contentHash</c> set to the placeholder,
+    /// then hashes the canonical form.
+    /// </summary>
+    /// <param name="manifest">The manifest document to hash.</param>
+    /// <returns>Hash in "sha256:&lt;hex&gt;" format.</returns>
+    public static string ComputeFromManifest(WeightManifestDocument manifest)
+    {
+        ArgumentNullException.ThrowIfNull(manifest);
+
+        // Serialize with placeholder to ensure contentHash doesn't affect the result
+        var withPlaceholder = manifest with
+        {
+            ContentHash = WeightManifestConstants.AutoHashPlaceholder
+        };
+
+        var json = JsonSerializer.Serialize(withPlaceholder, CanonicalOptions);
+        return ComputeFromJson(json);
+    }
+
+    /// <summary>
+    /// Verifies that a manifest's stored content hash matches its computed hash.
+    /// </summary>
+    /// <param name="jsonContent">Raw JSON content of the manifest file.</param>
+    /// <param name="storedHash">The hash stored in the manifest's contentHash field.</param>
+    /// <returns>True if the hashes match.</returns>
+    public static bool Verify(string jsonContent, string storedHash)
+    {
+        if (string.IsNullOrEmpty(storedHash)
+            || storedHash.Equals(WeightManifestConstants.AutoHashPlaceholder, StringComparison.Ordinal))
+        {
+            return false;
+        }
+
+        var computed = ComputeFromJson(jsonContent);
+        return computed.Equals(storedHash, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    /// Replaces the "sha256:auto" placeholder in raw JSON with the computed hash.
+    /// Returns the updated JSON content and the computed hash.
+    /// </summary>
+    /// <param name="jsonContent">Raw JSON with contentHash placeholder.</param>
+    /// <returns>Tuple of (updatedJson, computedHash).</returns>
+    public static (string UpdatedJson, string ComputedHash) ReplaceAutoHash(string jsonContent)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(jsonContent);
+
+        var computedHash = ComputeFromJson(jsonContent);
+
+        var updatedJson = jsonContent.Replace(
+            $"\"{WeightManifestConstants.AutoHashPlaceholder}\"",
+            $"\"{computedHash}\"",
+            StringComparison.Ordinal);
+
+        return (updatedJson, computedHash);
+    }
+
+    /// <summary>
+    /// Builds a canonical JSON string from a <see cref="JsonElement"/>,
+    /// excluding the <c>contentHash</c> field for hash stability.
+    /// Properties are sorted alphabetically for determinism.
+    /// </summary>
+    private static string BuildCanonicalContent(JsonElement root)
+    {
+        using var stream = new MemoryStream();
+        using var writer = new Utf8JsonWriter(stream, new JsonWriterOptions
+        {
+            Indented = false,
+            SkipValidation = false
+        });
+
+        WriteCanonical(writer, root, excludeField: "contentHash");
+        writer.Flush();
+
+        return Encoding.UTF8.GetString(stream.ToArray());
+    }
+
+    /// <summary>
+    /// Recursively writes JSON with sorted property keys and an optional excluded field.
+    /// </summary>
+    private static void WriteCanonical(Utf8JsonWriter writer, JsonElement element, string? excludeField = null)
+    {
+        switch (element.ValueKind)
+        {
+            case JsonValueKind.Object:
+                writer.WriteStartObject();
+
+                // Sort properties alphabetically for deterministic output
+                var properties = element.EnumerateObject()
+                    .Where(p => !string.Equals(p.Name, excludeField, StringComparison.Ordinal))
+                    .OrderBy(p => p.Name, StringComparer.Ordinal)
+                    .ToList();
+
+                foreach (var property in properties)
+                {
+                    writer.WritePropertyName(property.Name);
+                    WriteCanonical(writer, property.Value);
+                }
+
+                writer.WriteEndObject();
+                break;
+
+            case JsonValueKind.Array:
+                writer.WriteStartArray();
+                foreach (var item in element.EnumerateArray())
+                {
+                    WriteCanonical(writer, item);
+                }
+                writer.WriteEndArray();
+                break;
+
+            case JsonValueKind.String:
+                writer.WriteStringValue(element.GetString());
+                break;
+
+            case JsonValueKind.Number:
+                if (element.TryGetInt64(out var longValue))
+                    writer.WriteNumberValue(longValue);
+                else
+                    writer.WriteNumberValue(element.GetDouble());
+                break;
+
+            case JsonValueKind.True:
+                writer.WriteBooleanValue(true);
+                break;
+
+            case JsonValueKind.False:
+                writer.WriteBooleanValue(false);
+                break;
+
+            case JsonValueKind.Null:
+                writer.WriteNullValue();
+                break;
+        }
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/WeightManifest/WeightManifestLoader.cs

@@ -0,0 +1,403 @@
+// -----------------------------------------------------------------------------
+// WeightManifestLoader.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Weight manifest loader implementation
+// Description: File-system-based weight manifest discovery, loading,
+//              validation, selection by effectiveFrom date, and diffing.
+//              Deterministic and offline-friendly (no network calls).
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Diagnostics;
+using System.Diagnostics.Metrics;
+using System.Text.Json;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+/// <summary>
+/// Configuration options for the weight manifest loader.
+/// </summary>
+public sealed record WeightManifestLoaderOptions
+{
+    /// <summary>Configuration section name.</summary>
+    public const string SectionName = "Determinization:WeightManifest";
+
+    /// <summary>
+    /// Base directory to discover manifests in.
+    /// Defaults to <c>etc/weights</c> relative to the application root.
+    /// </summary>
+    public string ManifestDirectory { get; init; } = WeightManifestConstants.DefaultManifestDirectory;
+
+    /// <summary>
+    /// Glob pattern for manifest files.
+    /// </summary>
+    public string FilePattern { get; init; } = WeightManifestConstants.DefaultGlobPattern;
+
+    /// <summary>
+    /// Whether to require valid content hashes (reject "sha256:auto").
+    /// In production this should be true; in development, false is acceptable.
+    /// </summary>
+    public bool RequireComputedHash { get; init; }
+
+    /// <summary>
+    /// Whether to fail on hash mismatch (true) or log a warning (false).
+    /// </summary>
+    public bool StrictHashVerification { get; init; }
+}
+
+/// <summary>
+/// File-system-based weight manifest loader with deterministic behavior.
+/// Discovers manifests from a configured directory, validates them,
+/// computes/verifies content hashes, and selects by effectiveFrom date.
+/// </summary>
+public sealed class WeightManifestLoader : IWeightManifestLoader
+{
+    private static readonly Meter Meter = new("StellaOps.Policy.Determinization.WeightManifest", "1.0.0");
+    private static readonly Counter<long> ManifestsLoaded = Meter.CreateCounter<long>(
+        "stellaops.weight_manifest.loaded_total", "manifests", "Total manifests loaded");
+    private static readonly Counter<long> ManifestsValidated = Meter.CreateCounter<long>(
+        "stellaops.weight_manifest.validated_total", "manifests", "Total manifests validated");
+    private static readonly Counter<long> HashMismatches = Meter.CreateCounter<long>(
+        "stellaops.weight_manifest.hash_mismatch_total", "errors", "Content hash mismatches detected");
+    private static readonly Counter<long> ValidationErrors = Meter.CreateCounter<long>(
+        "stellaops.weight_manifest.validation_error_total", "errors", "Validation errors encountered");
+
+    private static readonly JsonSerializerOptions DeserializeOptions = new()
+    {
+        PropertyNameCaseInsensitive = true,
+        ReadCommentHandling = JsonCommentHandling.Skip,
+        AllowTrailingCommas = true
+    };
+
+    private readonly WeightManifestLoaderOptions _options;
+    private readonly ILogger<WeightManifestLoader> _logger;
+
+    public WeightManifestLoader(
+        IOptions<WeightManifestLoaderOptions> options,
+        ILogger<WeightManifestLoader> logger)
+    {
+        _options = options?.Value ?? throw new ArgumentNullException(nameof(options));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <inheritdoc />
+    public async Task<ImmutableArray<WeightManifestLoadResult>> ListAsync(
+        CancellationToken cancellationToken = default)
+    {
+        var directory = ResolveManifestDirectory();
+        if (!Directory.Exists(directory))
+        {
+            _logger.LogWarning("Weight manifest directory not found: {Directory}", directory);
+            return [];
+        }
+
+        var files = Directory.GetFiles(directory, _options.FilePattern)
+            .OrderBy(f => f, StringComparer.Ordinal)
+            .ToList();
+
+        if (files.Count == 0)
+        {
+            _logger.LogWarning("No weight manifest files found in {Directory}", directory);
+            return [];
+        }
+
+        var results = new List<WeightManifestLoadResult>(files.Count);
+        foreach (var file in files)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            try
+            {
+                var result = await LoadCoreAsync(file, cancellationToken).ConfigureAwait(false);
+                results.Add(result);
+            }
+            catch (WeightManifestLoadException ex)
+            {
+                _logger.LogWarning(ex, "Skipping invalid manifest: {File}", file);
+            }
+        }
+
+        // Sort by effectiveFrom descending (most recent first)
+        return [.. results.OrderByDescending(r => r.Manifest.EffectiveFrom)];
+    }
+
+    /// <inheritdoc />
+    public Task<WeightManifestLoadResult> LoadAsync(
+        string filePath,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(filePath);
+        return LoadCoreAsync(filePath, cancellationToken);
+    }
+
+    /// <inheritdoc />
+    public async Task<WeightManifestLoadResult?> SelectEffectiveAsync(
+        DateTimeOffset referenceDate,
+        CancellationToken cancellationToken = default)
+    {
+        var all = await ListAsync(cancellationToken).ConfigureAwait(false);
+        if (all.IsEmpty)
+            return null;
+
+        // Already sorted by effectiveFrom descending; pick first where effectiveFrom <= referenceDate
+        return all.FirstOrDefault(r => r.Manifest.EffectiveFrom <= referenceDate);
+    }
+
+    /// <inheritdoc />
+    public ImmutableArray<string> Validate(WeightManifestLoadResult result)
+    {
+        ArgumentNullException.ThrowIfNull(result);
+
+        var issues = new List<string>();
+        var manifest = result.Manifest;
+
+        // Schema version check
+        if (!string.Equals(manifest.SchemaVersion, WeightManifestConstants.SupportedSchemaVersion,
+                StringComparison.Ordinal))
+        {
+            issues.Add(
+                $"Unsupported schema version '{manifest.SchemaVersion}'. Expected '{WeightManifestConstants.SupportedSchemaVersion}'.");
+        }
+
+        // Version field
+        if (string.IsNullOrWhiteSpace(manifest.Version))
+        {
+            issues.Add("Version field is required.");
+        }
+
+        // Content hash
+        if (_options.RequireComputedHash && !manifest.HasComputedHash)
+        {
+            issues.Add("Content hash is required but manifest contains placeholder 'sha256:auto'.");
+        }
+
+        if (manifest.HasComputedHash && !result.HashVerified)
+        {
+            issues.Add(
+                $"Content hash mismatch: stored={manifest.ContentHash}, computed={result.ComputedHash}.");
+        }
+
+        // Legacy weight normalization
+        if (manifest.Weights.Legacy.Count > 0)
+        {
+            var legacySum = manifest.Weights.Legacy.Values.Sum();
+            if (Math.Abs(legacySum - 1.0) > 0.001)
+            {
+                issues.Add($"Legacy weights sum to {legacySum:F4}, expected 1.0.");
+            }
+        }
+
+        // Advisory weight normalization
+        if (manifest.Weights.Advisory.Count > 0)
+        {
+            var advisorySum = manifest.Weights.Advisory.Values.Sum();
+            if (Math.Abs(advisorySum - 1.0) > 0.001)
+            {
+                issues.Add($"Advisory weights sum to {advisorySum:F4}, expected 1.0.");
+            }
+        }
+
+        // Signal weights for entropy normalization
+        if (manifest.SignalWeightsForEntropy.Count > 0)
+        {
+            var signalSum = manifest.SignalWeightsForEntropy.Values.Sum();
+            if (Math.Abs(signalSum - 1.0) > 0.001)
+            {
+                issues.Add($"Signal weights for entropy sum to {signalSum:F4}, expected 1.0.");
+            }
+        }
+
+        ManifestsValidated.Add(1);
+        if (issues.Count > 0)
+        {
+            ValidationErrors.Add(issues.Count);
+        }
+
+        return [.. issues];
+    }
+
+    /// <inheritdoc />
+    public WeightManifestDiff Diff(WeightManifestDocument from, WeightManifestDocument to)
+    {
+        ArgumentNullException.ThrowIfNull(from);
+        ArgumentNullException.ThrowIfNull(to);
+
+        var diffs = new List<WeightManifestFieldDiff>();
+
+        // Compare scalar fields
+        CompareScalar(diffs, "version", from.Version, to.Version);
+        CompareScalar(diffs, "profile", from.Profile, to.Profile);
+        CompareScalar(diffs, "effectiveFrom", from.EffectiveFrom.ToString("O"), to.EffectiveFrom.ToString("O"));
+
+        // Compare legacy weights
+        CompareWeightDictionary(diffs, "weights.legacy", from.Weights.Legacy, to.Weights.Legacy);
+
+        // Compare advisory weights
+        CompareWeightDictionary(diffs, "weights.advisory", from.Weights.Advisory, to.Weights.Advisory);
+
+        // Compare signal weights for entropy
+        CompareWeightDictionary(diffs, "signalWeightsForEntropy",
+            from.SignalWeightsForEntropy, to.SignalWeightsForEntropy);
+
+        // Compare bucket thresholds
+        if (from.Buckets is not null && to.Buckets is not null)
+        {
+            CompareScalar(diffs, "buckets.actNowMin",
+                from.Buckets.ActNowMin.ToString(), to.Buckets.ActNowMin.ToString());
+            CompareScalar(diffs, "buckets.scheduleNextMin",
+                from.Buckets.ScheduleNextMin.ToString(), to.Buckets.ScheduleNextMin.ToString());
+            CompareScalar(diffs, "buckets.investigateMin",
+                from.Buckets.InvestigateMin.ToString(), to.Buckets.InvestigateMin.ToString());
+        }
+
+        // Compare determinization thresholds
+        if (from.DeterminizationThresholds is not null && to.DeterminizationThresholds is not null)
+        {
+            CompareScalar(diffs, "determinizationThresholds.manualReviewEntropy",
+                from.DeterminizationThresholds.ManualReviewEntropy.ToString("F4"),
+                to.DeterminizationThresholds.ManualReviewEntropy.ToString("F4"));
+            CompareScalar(diffs, "determinizationThresholds.refreshEntropy",
+                from.DeterminizationThresholds.RefreshEntropy.ToString("F4"),
+                to.DeterminizationThresholds.RefreshEntropy.ToString("F4"));
+        }
+
+        return new WeightManifestDiff
+        {
+            FromVersion = from.Version,
+            ToVersion = to.Version,
+            Differences = [.. diffs]
+        };
+    }
+
+    // ── Private helpers ──────────────────────────────────────────────────
+
+    private async Task<WeightManifestLoadResult> LoadCoreAsync(
+        string filePath,
+        CancellationToken cancellationToken)
+    {
+        var resolvedPath = Path.GetFullPath(filePath);
+        if (!File.Exists(resolvedPath))
+        {
+            throw new WeightManifestLoadException($"Weight manifest file not found: {resolvedPath}");
+        }
+
+        var json = await File.ReadAllTextAsync(resolvedPath, cancellationToken).ConfigureAwait(false);
+
+        WeightManifestDocument manifest;
+        try
+        {
+            manifest = JsonSerializer.Deserialize<WeightManifestDocument>(json, DeserializeOptions)
+                ?? throw new WeightManifestLoadException(
+                    $"Failed to deserialize weight manifest from {resolvedPath}: empty document");
+        }
+        catch (JsonException ex)
+        {
+            throw new WeightManifestLoadException(
+                $"JSON parse error in {resolvedPath}: {ex.Message}", ex);
+        }
+
+        // Compute content hash
+        var computedHash = WeightManifestHashComputer.ComputeFromJson(json);
+        var hashVerified = manifest.HasComputedHash
+            && computedHash.Equals(manifest.ContentHash, StringComparison.Ordinal);
+
+        if (manifest.HasComputedHash && !hashVerified)
+        {
+            HashMismatches.Add(1);
+            var message =
+                $"Content hash mismatch for {resolvedPath}: stored={manifest.ContentHash}, computed={computedHash}";
+
+            if (_options.StrictHashVerification)
+            {
+                throw new WeightManifestLoadException(message);
+            }
+
+            _logger.LogWarning("{Message}", message);
+        }
+
+        ManifestsLoaded.Add(1);
+
+        _logger.LogDebug(
+            "Loaded weight manifest {Version} from {Path} (hash verified: {HashVerified})",
+            manifest.Version, resolvedPath, hashVerified);
+
+        return new WeightManifestLoadResult
+        {
+            Manifest = manifest,
+            SourcePath = resolvedPath,
+            HashVerified = hashVerified,
+            ComputedHash = computedHash
+        };
+    }
+
+    private string ResolveManifestDirectory()
+    {
+        var dir = _options.ManifestDirectory;
+        if (Path.IsPathRooted(dir))
+            return dir;
+
+        // Resolve relative to current directory (application root)
+        return Path.GetFullPath(dir, AppContext.BaseDirectory);
+    }
+
+    private static void CompareScalar(
+        List<WeightManifestFieldDiff> diffs, string path, string? from, string? to)
+    {
+        if (!string.Equals(from, to, StringComparison.Ordinal))
+        {
+            diffs.Add(new WeightManifestFieldDiff { Path = path, OldValue = from, NewValue = to });
+        }
+    }
+
+    private static void CompareWeightDictionary(
+        List<WeightManifestFieldDiff> diffs,
+        string prefix,
+        ImmutableDictionary<string, double> from,
+        ImmutableDictionary<string, double> to)
+    {
+        var allKeys = from.Keys.Union(to.Keys).Order().ToList();
+        foreach (var key in allKeys)
+        {
+            var hasFrom = from.TryGetValue(key, out var fromVal);
+            var hasTo = to.TryGetValue(key, out var toVal);
+
+            if (!hasFrom)
+            {
+                diffs.Add(new WeightManifestFieldDiff
+                {
+                    Path = $"{prefix}.{key}",
+                    OldValue = null,
+                    NewValue = toVal.ToString("F4")
+                });
+            }
+            else if (!hasTo)
+            {
+                diffs.Add(new WeightManifestFieldDiff
+                {
+                    Path = $"{prefix}.{key}",
+                    OldValue = fromVal.ToString("F4"),
+                    NewValue = null
+                });
+            }
+            else if (Math.Abs(fromVal - toVal) > 0.0001)
+            {
+                diffs.Add(new WeightManifestFieldDiff
+                {
+                    Path = $"{prefix}.{key}",
+                    OldValue = fromVal.ToString("F4"),
+                    NewValue = toVal.ToString("F4")
+                });
+            }
+        }
+    }
+}
+
+/// <summary>
+/// Exception thrown when weight manifest loading or validation fails.
+/// </summary>
+public sealed class WeightManifestLoadException : Exception
+{
+    public WeightManifestLoadException(string message) : base(message) { }
+    public WeightManifestLoadException(string message, Exception inner) : base(message, inner) { }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/Scoring/WeightManifest/WeightManifestModels.cs

@@ -0,0 +1,278 @@
+// -----------------------------------------------------------------------------
+// WeightManifestModels.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Versioned weight manifest models
+// Description: Immutable models for weight manifests with content-addressed
+//              hashing, versioning, and deterministic serialization.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+/// <summary>
+/// Immutable representation of a versioned weight manifest file.
+/// </summary>
+public sealed record WeightManifestDocument
+{
+    /// <summary>JSON Schema URI.</summary>
+    [JsonPropertyName("$schema")]
+    public string? Schema { get; init; }
+
+    /// <summary>Schema version (e.g. "1.0.0").</summary>
+    [JsonPropertyName("schemaVersion")]
+    public required string SchemaVersion { get; init; }
+
+    /// <summary>Manifest version identifier (e.g. "v2026-01-22").</summary>
+    [JsonPropertyName("version")]
+    public required string Version { get; init; }
+
+    /// <summary>UTC date from which this manifest is effective.</summary>
+    [JsonPropertyName("effectiveFrom")]
+    public required DateTimeOffset EffectiveFrom { get; init; }
+
+    /// <summary>Profile name (e.g. "production", "staging").</summary>
+    [JsonPropertyName("profile")]
+    public string Profile { get; init; } = "production";
+
+    /// <summary>Human-readable description of this manifest.</summary>
+    [JsonPropertyName("description")]
+    public string? Description { get; init; }
+
+    /// <summary>
+    /// Content hash in "sha256:&lt;hex&gt;" format.
+    /// The placeholder "sha256:auto" means the hash has not been computed yet.
+    /// </summary>
+    [JsonPropertyName("contentHash")]
+    public required string ContentHash { get; init; }
+
+    /// <summary>Legacy 6-dimension EWS weights.</summary>
+    [JsonPropertyName("weights")]
+    public required WeightManifestWeights Weights { get; init; }
+
+    /// <summary>Dimension human-readable names.</summary>
+    [JsonPropertyName("dimensionNames")]
+    public ImmutableDictionary<string, string> DimensionNames { get; init; } =
+        ImmutableDictionary<string, string>.Empty;
+
+    /// <summary>Dimensions that subtract from risk score.</summary>
+    [JsonPropertyName("subtractiveDimensions")]
+    public ImmutableArray<string> SubtractiveDimensions { get; init; } = [];
+
+    /// <summary>Guardrail configurations.</summary>
+    [JsonPropertyName("guardrails")]
+    public WeightManifestGuardrails? Guardrails { get; init; }
+
+    /// <summary>Bucket boundaries for action tiers.</summary>
+    [JsonPropertyName("buckets")]
+    public WeightManifestBuckets? Buckets { get; init; }
+
+    /// <summary>Determinization thresholds for entropy-based triage.</summary>
+    [JsonPropertyName("determinizationThresholds")]
+    public WeightManifestDeterminizationThresholds? DeterminizationThresholds { get; init; }
+
+    /// <summary>Signal weights for entropy calculation (maps to <see cref="SignalWeights"/>).</summary>
+    [JsonPropertyName("signalWeightsForEntropy")]
+    public ImmutableDictionary<string, double> SignalWeightsForEntropy { get; init; } =
+        ImmutableDictionary<string, double>.Empty;
+
+    /// <summary>Provenance metadata.</summary>
+    [JsonPropertyName("metadata")]
+    public WeightManifestMetadata? Metadata { get; init; }
+
+    /// <summary>Whether the content hash is a computed hash vs. the placeholder.</summary>
+    [JsonIgnore]
+    public bool HasComputedHash => !string.IsNullOrEmpty(ContentHash)
+        && !ContentHash.Equals(WeightManifestConstants.AutoHashPlaceholder, StringComparison.Ordinal);
+}
+
+/// <summary>
+/// Multi-profile weights block (legacy + advisory).
+/// </summary>
+public sealed record WeightManifestWeights
+{
+    /// <summary>Legacy 6-dimension weights.</summary>
+    [JsonPropertyName("legacy")]
+    public ImmutableDictionary<string, double> Legacy { get; init; } =
+        ImmutableDictionary<string, double>.Empty;
+
+    /// <summary>Advisory weights.</summary>
+    [JsonPropertyName("advisory")]
+    public ImmutableDictionary<string, double> Advisory { get; init; } =
+        ImmutableDictionary<string, double>.Empty;
+}
+
+/// <summary>
+/// Guardrail configuration from a weight manifest.
+/// </summary>
+public sealed record WeightManifestGuardrails
+{
+    [JsonPropertyName("notAffectedCap")]
+    public GuardrailRule? NotAffectedCap { get; init; }
+
+    [JsonPropertyName("runtimeFloor")]
+    public GuardrailRule? RuntimeFloor { get; init; }
+
+    [JsonPropertyName("speculativeCap")]
+    public GuardrailRule? SpeculativeCap { get; init; }
+}
+
+/// <summary>
+/// Individual guardrail rule.
+/// </summary>
+public sealed record GuardrailRule
+{
+    [JsonPropertyName("enabled")]
+    public bool Enabled { get; init; }
+
+    [JsonPropertyName("maxScore")]
+    public int? MaxScore { get; init; }
+
+    [JsonPropertyName("minScore")]
+    public int? MinScore { get; init; }
+
+    [JsonPropertyName("requiresBkpMin")]
+    public double? RequiresBkpMin { get; init; }
+
+    [JsonPropertyName("requiresRtsMax")]
+    public double? RequiresRtsMax { get; init; }
+
+    [JsonPropertyName("requiresRtsMin")]
+    public double? RequiresRtsMin { get; init; }
+
+    [JsonPropertyName("requiresRchMax")]
+    public double? RequiresRchMax { get; init; }
+}
+
+/// <summary>
+/// Action bucket boundaries.
+/// </summary>
+public sealed record WeightManifestBuckets
+{
+    [JsonPropertyName("actNowMin")]
+    public int ActNowMin { get; init; } = 90;
+
+    [JsonPropertyName("scheduleNextMin")]
+    public int ScheduleNextMin { get; init; } = 70;
+
+    [JsonPropertyName("investigateMin")]
+    public int InvestigateMin { get; init; } = 40;
+}
+
+/// <summary>
+/// Entropy-based determinization thresholds.
+/// </summary>
+public sealed record WeightManifestDeterminizationThresholds
+{
+    [JsonPropertyName("manualReviewEntropy")]
+    public double ManualReviewEntropy { get; init; } = 0.60;
+
+    [JsonPropertyName("refreshEntropy")]
+    public double RefreshEntropy { get; init; } = 0.40;
+}
+
+/// <summary>
+/// Provenance metadata for audit trail.
+/// </summary>
+public sealed record WeightManifestMetadata
+{
+    [JsonPropertyName("createdBy")]
+    public string? CreatedBy { get; init; }
+
+    [JsonPropertyName("createdAt")]
+    public DateTimeOffset? CreatedAt { get; init; }
+
+    [JsonPropertyName("changelog")]
+    public ImmutableArray<ChangelogEntry> Changelog { get; init; } = [];
+
+    [JsonPropertyName("notes")]
+    public ImmutableArray<string> Notes { get; init; } = [];
+}
+
+/// <summary>
+/// Changelog entry for manifest versioning audit.
+/// </summary>
+public sealed record ChangelogEntry
+{
+    [JsonPropertyName("version")]
+    public string? Version { get; init; }
+
+    [JsonPropertyName("date")]
+    public string? Date { get; init; }
+
+    [JsonPropertyName("changes")]
+    public ImmutableArray<string> Changes { get; init; } = [];
+}
+
+/// <summary>
+/// Constants for the weight manifest system.
+/// </summary>
+public static class WeightManifestConstants
+{
+    /// <summary>Placeholder that signals "compute hash at build/load time".</summary>
+    public const string AutoHashPlaceholder = "sha256:auto";
+
+    /// <summary>Prefix for content hashes.</summary>
+    public const string HashPrefix = "sha256:";
+
+    /// <summary>Supported schema version.</summary>
+    public const string SupportedSchemaVersion = "1.0.0";
+
+    /// <summary>Default glob pattern for discovering manifest files.</summary>
+    public const string DefaultGlobPattern = "*.weights.json";
+
+    /// <summary>Default manifest directory (relative to application root).</summary>
+    public const string DefaultManifestDirectory = "etc/weights";
+}
+
+/// <summary>
+/// Result of loading and validating a weight manifest.
+/// </summary>
+public sealed record WeightManifestLoadResult
+{
+    /// <summary>The loaded and validated manifest.</summary>
+    public required WeightManifestDocument Manifest { get; init; }
+
+    /// <summary>File path the manifest was loaded from.</summary>
+    public required string SourcePath { get; init; }
+
+    /// <summary>Whether the content hash was verified (vs computed fresh).</summary>
+    public required bool HashVerified { get; init; }
+
+    /// <summary>Computed content hash (may differ from manifest if auto).</summary>
+    public required string ComputedHash { get; init; }
+}
+
+/// <summary>
+/// Result of comparing two weight manifests.
+/// </summary>
+public sealed record WeightManifestDiff
+{
+    /// <summary>Source (older) manifest version.</summary>
+    public required string FromVersion { get; init; }
+
+    /// <summary>Target (newer) manifest version.</summary>
+    public required string ToVersion { get; init; }
+
+    /// <summary>Individual field differences.</summary>
+    public required ImmutableArray<WeightManifestFieldDiff> Differences { get; init; }
+
+    /// <summary>Whether any differences exist.</summary>
+    public bool HasDifferences => !Differences.IsEmpty;
+}
+
+/// <summary>
+/// Individual field difference between two manifests.
+/// </summary>
+public sealed record WeightManifestFieldDiff
+{
+    /// <summary>Dot-delimited path to the changed field.</summary>
+    public required string Path { get; init; }
+
+    /// <summary>Previous value (serialized as string).</summary>
+    public required string? OldValue { get; init; }
+
+    /// <summary>New value (serialized as string).</summary>
+    public required string? NewValue { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Determinization/ServiceCollectionExtensions.cs

@@ -3,6 +3,9 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Options;
 using StellaOps.Policy.Determinization.Scoring;
+using StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+using StellaOps.Policy.Determinization.Scoring.Triage;
+using StellaOps.Policy.Determinization.Scoring.WeightManifest;
 
 namespace StellaOps.Policy.Determinization;
 
@@ -43,8 +46,26 @@ public static class ServiceCollectionExtensions
         services.TryAddSingleton<DecayedConfidenceCalculator>();
         services.TryAddSingleton<IDecayedConfidenceCalculator>(sp => sp.GetRequiredService<DecayedConfidenceCalculator>());
 
+        services.TryAddSingleton<ImpactScoreCalculator>();
+        services.TryAddSingleton<IImpactScoreCalculator>(sp => sp.GetRequiredService<ImpactScoreCalculator>());
+
+        services.TryAddSingleton<CombinedImpactCalculator>();
+        services.TryAddSingleton<ICombinedImpactCalculator>(sp => sp.GetRequiredService<CombinedImpactCalculator>());
+
         services.TryAddSingleton<TrustScoreAggregator>();
 
+        services.TryAddSingleton<TrustScoreAlgebraFacade>();
+        services.TryAddSingleton<ITrustScoreAlgebraFacade>(sp => sp.GetRequiredService<TrustScoreAlgebraFacade>());
+
+        // EWS: 6-dimension Evidence-Weighted Score model
+        RegisterEwsServices(services);
+
+        // Triage: decay-based staleness evaluation and re-analysis queue
+        RegisterTriageServices(services);
+
+        // Weight Manifests: versioned weight discovery, validation, selection
+        RegisterWeightManifestServices(services);
+
         return services;
     }
 
@@ -66,8 +87,76 @@ public static class ServiceCollectionExtensions
         services.TryAddSingleton<DecayedConfidenceCalculator>();
         services.TryAddSingleton<IDecayedConfidenceCalculator>(sp => sp.GetRequiredService<DecayedConfidenceCalculator>());
 
+        services.TryAddSingleton<ImpactScoreCalculator>();
+        services.TryAddSingleton<IImpactScoreCalculator>(sp => sp.GetRequiredService<ImpactScoreCalculator>());
+
+        services.TryAddSingleton<CombinedImpactCalculator>();
+        services.TryAddSingleton<ICombinedImpactCalculator>(sp => sp.GetRequiredService<CombinedImpactCalculator>());
+
         services.TryAddSingleton<TrustScoreAggregator>();
 
+        services.TryAddSingleton<TrustScoreAlgebraFacade>();
+        services.TryAddSingleton<ITrustScoreAlgebraFacade>(sp => sp.GetRequiredService<TrustScoreAlgebraFacade>());
+
+        // TSF-004: Delta-if-present calculator for hypothetical score simulations
+        services.TryAddSingleton<DeltaIfPresentCalculator>();
+        services.TryAddSingleton<IDeltaIfPresentCalculator>(sp => sp.GetRequiredService<DeltaIfPresentCalculator>());
+
+        // EWS: 6-dimension Evidence-Weighted Score model
+        RegisterEwsServices(services);
+
+        // Triage: decay-based staleness evaluation and re-analysis queue
+        RegisterTriageServices(services);
+
+        // Weight Manifests: versioned weight discovery, validation, selection
+        RegisterWeightManifestServices(services);
+
         return services;
     }
+
+    private static void RegisterEwsServices(IServiceCollection services)
+    {
+        // Register all 6 dimension normalizers (AddSingleton, not TryAdd,
+        // so IEnumerable<IEwsDimensionNormalizer> resolves all of them)
+        services.AddSingleton<IEwsDimensionNormalizer, ReachabilityNormalizer>();
+        services.AddSingleton<IEwsDimensionNormalizer, RuntimeSignalsNormalizer>();
+        services.AddSingleton<IEwsDimensionNormalizer, BackportEvidenceNormalizer>();
+        services.AddSingleton<IEwsDimensionNormalizer, ExploitabilityNormalizer>();
+        services.AddSingleton<IEwsDimensionNormalizer, SourceConfidenceNormalizer>();
+        services.AddSingleton<IEwsDimensionNormalizer, MitigationStatusNormalizer>();
+
+        // Register guardrails engine
+        services.TryAddSingleton<IGuardrailsEngine, GuardrailsEngine>();
+
+        // Register unified EWS calculator
+        services.TryAddSingleton<IEwsCalculator, EwsCalculator>();
+    }
+
+    private static void RegisterTriageServices(IServiceCollection services)
+    {
+        // Register triage options (defaults if not bound to config)
+        services.AddOptions<TriageQueueOptions>();
+
+        // Register evaluator
+        services.TryAddSingleton<ITriageQueueEvaluator, TriageQueueEvaluator>();
+
+        // Register in-memory sink as default (can be overridden by host-level registration)
+        services.TryAddSingleton<InMemoryTriageReanalysisSink>();
+        services.TryAddSingleton<ITriageReanalysisSink>(sp => sp.GetRequiredService<InMemoryTriageReanalysisSink>());
+
+        // Register the triage queue service
+        services.TryAddSingleton<UnknownTriageQueueService>();
+    }
+
+    private static void RegisterWeightManifestServices(IServiceCollection services)
+    {
+        // Register loader options (defaults if not bound to config)
+        services.AddOptions<WeightManifestLoaderOptions>();
+
+        // Register manifest loader
+        services.TryAddSingleton<IWeightManifestLoader, WeightManifestLoader>();
+
+        // Register CLI command service
+        services.TryAddSingleton<WeightManifestCommands>();
+    }
 }

src/Policy/__Libraries/StellaOps.Policy.Explainability/ProofGraphBuilder.cs

@@ -0,0 +1,452 @@
+// -----------------------------------------------------------------------------
+// ProofGraphBuilder.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T1 - Proof graph builder
+// Description: Constructs proof graphs from verdict rationale data.
+//              Deterministic: same inputs always produce same graph with
+//              content-addressed ID. Supports counterfactual overlay nodes.
+// -----------------------------------------------------------------------------
+
+using System.Security.Cryptography;
+
+namespace StellaOps.Policy.Explainability;
+
+/// <summary>
+/// Builds proof graphs from verdict rationale components.
+/// </summary>
+public interface IProofGraphBuilder
+{
+    /// <summary>
+    /// Builds a complete proof graph from a verdict rationale and
+    /// optional score breakdown data.
+    /// </summary>
+    ProofGraph Build(ProofGraphInput input);
+
+    /// <summary>
+    /// Adds a counterfactual overlay to an existing proof graph,
+    /// showing how scores would change under hypothetical conditions.
+    /// </summary>
+    ProofGraph AddCounterfactualOverlay(
+        ProofGraph baseGraph,
+        CounterfactualScenario scenario);
+}
+
+/// <summary>
+/// Input data for building a proof graph.
+/// </summary>
+public sealed record ProofGraphInput
+{
+    /// <summary>The verdict rationale to visualize.</summary>
+    public required VerdictRationale Rationale { get; init; }
+
+    /// <summary>Per-factor score breakdown, if available.</summary>
+    public ScoreBreakdownDashboard? ScoreBreakdown { get; init; }
+
+    /// <summary>Reference time for graph computation.</summary>
+    public required DateTimeOffset ComputedAt { get; init; }
+}
+
+/// <summary>
+/// A counterfactual scenario for what-if analysis.
+/// </summary>
+public sealed record CounterfactualScenario
+{
+    /// <summary>Scenario label.</summary>
+    [JsonPropertyName("label")]
+    public required string Label { get; init; }
+
+    /// <summary>Factor overrides (factorId → hypothetical score).</summary>
+    [JsonPropertyName("factor_overrides")]
+    public required ImmutableDictionary<string, int> FactorOverrides { get; init; }
+
+    /// <summary>Resulting composite score under this scenario.</summary>
+    [JsonPropertyName("resulting_score")]
+    public int? ResultingScore { get; init; }
+}
+
+/// <summary>
+/// Deterministic proof graph builder.
+/// </summary>
+public sealed class ProofGraphBuilder : IProofGraphBuilder
+{
+    private readonly ILogger<ProofGraphBuilder> _logger;
+
+    public ProofGraphBuilder(ILogger<ProofGraphBuilder> logger)
+    {
+        _logger = logger;
+    }
+
+    public ProofGraph Build(ProofGraphInput input)
+    {
+        ArgumentNullException.ThrowIfNull(input);
+
+        var nodes = new List<ProofGraphNode>();
+        var edges = new List<ProofGraphEdge>();
+
+        // 1. Create verdict root node (depth 0)
+        var verdictNodeId = $"verdict:{input.Rationale.VerdictRef.AttestationId}";
+        nodes.Add(new ProofGraphNode
+        {
+            Id = verdictNodeId,
+            Label = $"Verdict: {input.Rationale.Decision.Verdict}",
+            Type = ProofNodeType.Verdict,
+            Confidence = input.Rationale.Decision.Score.HasValue
+                ? input.Rationale.Decision.Score.Value / 100.0
+                : null,
+            ScoreContribution = input.Rationale.Decision.Score,
+            Depth = 0
+        });
+
+        // 2. Create policy rule node (depth 1)
+        var policyNodeId = $"policy:{input.Rationale.PolicyClause.ClauseId}";
+        nodes.Add(new ProofGraphNode
+        {
+            Id = policyNodeId,
+            Label = input.Rationale.PolicyClause.RuleDescription,
+            Type = ProofNodeType.PolicyRule,
+            Depth = 1
+        });
+        edges.Add(new ProofGraphEdge
+        {
+            Source = policyNodeId,
+            Target = verdictNodeId,
+            Relation = ProofEdgeRelation.Gates,
+            Label = "Policy evaluation"
+        });
+
+        // 3. Create score computation nodes from breakdown (depth 2)
+        if (input.ScoreBreakdown is not null)
+        {
+            foreach (var factor in input.ScoreBreakdown.Factors)
+            {
+                var factorNodeId = $"score:{factor.FactorId}";
+                nodes.Add(new ProofGraphNode
+                {
+                    Id = factorNodeId,
+                    Label = $"{factor.FactorName} ({factor.RawScore})",
+                    Type = ProofNodeType.ScoreComputation,
+                    Confidence = factor.Confidence,
+                    ScoreContribution = factor.WeightedContribution,
+                    Depth = 2,
+                    Metadata = ImmutableDictionary<string, string>.Empty
+                        .Add("weight", factor.Weight.ToString("F2"))
+                        .Add("raw_score", factor.RawScore.ToString())
+                });
+                edges.Add(new ProofGraphEdge
+                {
+                    Source = factorNodeId,
+                    Target = verdictNodeId,
+                    Relation = ProofEdgeRelation.ContributesScore,
+                    Weight = factor.Weight,
+                    Label = $"{factor.Weight:P0} weight"
+                });
+            }
+
+            // 3b. Guardrail nodes (depth 1, override verdict)
+            foreach (var guardrail in input.ScoreBreakdown.GuardrailsApplied)
+            {
+                var guardrailNodeId = $"guardrail:{guardrail.GuardrailName}";
+                nodes.Add(new ProofGraphNode
+                {
+                    Id = guardrailNodeId,
+                    Label = $"Guardrail: {guardrail.GuardrailName} ({guardrail.ScoreBefore}→{guardrail.ScoreAfter})",
+                    Type = ProofNodeType.Guardrail,
+                    Depth = 1,
+                    Metadata = ImmutableDictionary<string, string>.Empty
+                        .Add("reason", guardrail.Reason)
+                });
+                edges.Add(new ProofGraphEdge
+                {
+                    Source = guardrailNodeId,
+                    Target = verdictNodeId,
+                    Relation = ProofEdgeRelation.GuardrailApplied,
+                    Label = guardrail.Reason
+                });
+            }
+        }
+
+        // 4. Create evidence leaf nodes (depth 3)
+        var leafNodeIds = new List<string>();
+
+        // Reachability evidence
+        if (input.Rationale.Evidence.Reachability is not null)
+        {
+            var reachNodeId = $"evidence:reachability:{input.Rationale.Evidence.Cve}";
+            nodes.Add(new ProofGraphNode
+            {
+                Id = reachNodeId,
+                Label = $"Reachability: {input.Rationale.Evidence.Reachability.VulnerableFunction ?? "analyzed"}",
+                Type = ProofNodeType.ReachabilityAnalysis,
+                Depth = 3,
+                Metadata = ImmutableDictionary<string, string>.Empty
+                    .Add("entry_point", input.Rationale.Evidence.Reachability.EntryPoint ?? "unknown")
+            });
+            edges.Add(new ProofGraphEdge
+            {
+                Source = reachNodeId,
+                Target = TryFindScoreNode(nodes, "rch") ?? policyNodeId,
+                Relation = ProofEdgeRelation.ProvidesEvidence,
+                Label = "Reachability signal"
+            });
+            leafNodeIds.Add(reachNodeId);
+        }
+
+        // VEX statement evidence
+        if (input.Rationale.Attestations.VexStatements?.Count > 0)
+        {
+            for (int i = 0; i < input.Rationale.Attestations.VexStatements.Count; i++)
+            {
+                var vex = input.Rationale.Attestations.VexStatements[i];
+                var vexNodeId = $"evidence:vex:{vex.Id}";
+                nodes.Add(new ProofGraphNode
+                {
+                    Id = vexNodeId,
+                    Label = $"VEX: {vex.Summary ?? vex.Id}",
+                    Type = ProofNodeType.VexStatement,
+                    Digest = vex.Digest,
+                    Depth = 3
+                });
+                edges.Add(new ProofGraphEdge
+                {
+                    Source = vexNodeId,
+                    Target = policyNodeId,
+                    Relation = ProofEdgeRelation.Attests,
+                    Label = "VEX statement"
+                });
+                leafNodeIds.Add(vexNodeId);
+            }
+        }
+
+        // Provenance attestation
+        if (input.Rationale.Attestations.Provenance is not null)
+        {
+            var provNodeId = $"evidence:provenance:{input.Rationale.Attestations.Provenance.Id}";
+            nodes.Add(new ProofGraphNode
+            {
+                Id = provNodeId,
+                Label = $"Provenance: {input.Rationale.Attestations.Provenance.Summary ?? "verified"}",
+                Type = ProofNodeType.Provenance,
+                Digest = input.Rationale.Attestations.Provenance.Digest,
+                Depth = 3
+            });
+            edges.Add(new ProofGraphEdge
+            {
+                Source = provNodeId,
+                Target = policyNodeId,
+                Relation = ProofEdgeRelation.Attests,
+                Label = "Provenance attestation"
+            });
+            leafNodeIds.Add(provNodeId);
+        }
+
+        // Path witness
+        if (input.Rationale.Attestations.PathWitness is not null)
+        {
+            var pathNodeId = $"evidence:pathwitness:{input.Rationale.Attestations.PathWitness.Id}";
+            nodes.Add(new ProofGraphNode
+            {
+                Id = pathNodeId,
+                Label = $"Path Witness: {input.Rationale.Attestations.PathWitness.Summary ?? "verified"}",
+                Type = ProofNodeType.ReachabilityAnalysis,
+                Digest = input.Rationale.Attestations.PathWitness.Digest,
+                Depth = 3
+            });
+            edges.Add(new ProofGraphEdge
+            {
+                Source = pathNodeId,
+                Target = TryFindScoreNode(nodes, "rch") ?? policyNodeId,
+                Relation = ProofEdgeRelation.Attests,
+                Label = "Path witness attestation"
+            });
+            leafNodeIds.Add(pathNodeId);
+        }
+
+        // 5. Build critical paths (leaf → root)
+        var criticalPaths = BuildCriticalPaths(nodes, edges, verdictNodeId, leafNodeIds);
+
+        // 6. Compute content-addressed graph ID
+        var graphId = ComputeGraphId(nodes, edges);
+
+        var graph = new ProofGraph
+        {
+            GraphId = graphId,
+            VerdictRef = input.Rationale.VerdictRef,
+            Nodes = [.. nodes],
+            Edges = [.. edges],
+            CriticalPaths = [.. criticalPaths],
+            RootNodeId = verdictNodeId,
+            LeafNodeIds = [.. leafNodeIds],
+            ComputedAt = input.ComputedAt
+        };
+
+        _logger.LogDebug(
+            "Built proof graph {GraphId} with {NodeCount} nodes, {EdgeCount} edges, {PathCount} paths",
+            graphId, nodes.Count, edges.Count, criticalPaths.Count);
+
+        return graph;
+    }
+
+    public ProofGraph AddCounterfactualOverlay(
+        ProofGraph baseGraph,
+        CounterfactualScenario scenario)
+    {
+        ArgumentNullException.ThrowIfNull(baseGraph);
+        ArgumentNullException.ThrowIfNull(scenario);
+
+        var nodes = baseGraph.Nodes.ToList();
+        var edges = baseGraph.Edges.ToList();
+
+        // Add a counterfactual hypothesis node
+        var cfNodeId = $"counterfactual:{scenario.Label.Replace(" ", "_").ToLowerInvariant()}";
+        nodes.Add(new ProofGraphNode
+        {
+            Id = cfNodeId,
+            Label = $"What-If: {scenario.Label}",
+            Type = ProofNodeType.Counterfactual,
+            ScoreContribution = scenario.ResultingScore,
+            Depth = 0,
+            Metadata = scenario.FactorOverrides
+                .ToImmutableDictionary(kv => $"override_{kv.Key}", kv => kv.Value.ToString())
+        });
+
+        // Connect overridden factors to the counterfactual node
+        foreach (var (factorId, _) in scenario.FactorOverrides)
+        {
+            var existingNode = nodes.FirstOrDefault(n => n.Id == $"score:{factorId}");
+            if (existingNode is not null)
+            {
+                edges.Add(new ProofGraphEdge
+                {
+                    Source = existingNode.Id,
+                    Target = cfNodeId,
+                    Relation = ProofEdgeRelation.Overrides,
+                    Label = $"What-if override: {factorId}"
+                });
+            }
+        }
+
+        var newGraphId = ComputeGraphId(nodes, edges);
+
+        return baseGraph with
+        {
+            GraphId = newGraphId,
+            Nodes = [.. nodes],
+            Edges = [.. edges]
+        };
+    }
+
+    // ── Private helpers ──────────────────────────────────────────────────
+
+    private static string? TryFindScoreNode(List<ProofGraphNode> nodes, string factorCode)
+    {
+        return nodes.FirstOrDefault(n => n.Id == $"score:{factorCode}")?.Id;
+    }
+
+    private static List<ProofGraphPath> BuildCriticalPaths(
+        List<ProofGraphNode> nodes,
+        List<ProofGraphEdge> edges,
+        string rootId,
+        List<string> leafIds)
+    {
+        var paths = new List<ProofGraphPath>();
+
+        // Build adjacency list (reverse: from target to source for tracing back)
+        var reverseAdj = edges
+            .GroupBy(e => e.Target)
+            .ToDictionary(g => g.Key, g => g.Select(e => (e.Source, e.Weight)).ToList());
+
+        // Forward adjacency for tracing leaf to root
+        var forwardAdj = edges
+            .GroupBy(e => e.Source)
+            .ToDictionary(g => g.Key, g => g.Select(e => (e.Target, e.Weight)).ToList());
+
+        foreach (var leafId in leafIds)
+        {
+            var path = FindPathBfs(forwardAdj, leafId, rootId);
+            if (path.Count > 0)
+            {
+                // Calculate path confidence as product of edge weights
+                var confidence = 1.0;
+                for (int i = 0; i < path.Count - 1; i++)
+                {
+                    var edge = edges.FirstOrDefault(e =>
+                        e.Source == path[i] && e.Target == path[i + 1]);
+                    if (edge is not null)
+                    {
+                        confidence *= edge.Weight;
+                    }
+                }
+
+                var leafNode = nodes.FirstOrDefault(n => n.Id == leafId);
+                paths.Add(new ProofGraphPath
+                {
+                    NodeIds = [.. path],
+                    PathConfidence = confidence,
+                    Description = $"{leafNode?.Label ?? leafId} → verdict"
+                });
+            }
+        }
+
+        // Mark highest-confidence path as critical
+        if (paths.Count > 0)
+        {
+            var maxConfidence = paths.Max(p => p.PathConfidence);
+            for (int i = 0; i < paths.Count; i++)
+            {
+                if (Math.Abs(paths[i].PathConfidence - maxConfidence) < 0.0001)
+                {
+                    paths[i] = paths[i] with { IsCritical = true };
+                }
+            }
+        }
+
+        return paths;
+    }
+
+    private static List<string> FindPathBfs(
+        Dictionary<string, List<(string Target, double Weight)>> adj,
+        string from,
+        string to)
+    {
+        var visited = new HashSet<string>();
+        var queue = new Queue<List<string>>();
+        queue.Enqueue([from]);
+
+        while (queue.Count > 0)
+        {
+            var path = queue.Dequeue();
+            var current = path[^1];
+
+            if (current == to)
+                return path;
+
+            if (!visited.Add(current))
+                continue;
+
+            if (adj.TryGetValue(current, out var neighbors))
+            {
+                foreach (var (target, _) in neighbors.OrderBy(n => n.Target, StringComparer.Ordinal))
+                {
+                    if (!visited.Contains(target))
+                    {
+                        queue.Enqueue([.. path, target]);
+                    }
+                }
+            }
+        }
+
+        return [];
+    }
+
+    private static string ComputeGraphId(List<ProofGraphNode> nodes, List<ProofGraphEdge> edges)
+    {
+        // Deterministic: sort nodes by ID, edges by source+target
+        var sortedNodes = string.Join("|", nodes.OrderBy(n => n.Id).Select(n => n.Id));
+        var sortedEdges = string.Join("|", edges
+            .OrderBy(e => e.Source).ThenBy(e => e.Target)
+            .Select(e => $"{e.Source}->{e.Target}"));
+        var content = $"{sortedNodes}:{sortedEdges}";
+        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(content));
+        return $"pg:sha256:{Convert.ToHexStringLower(hash)}";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Explainability/ProofGraphModels.cs

@@ -0,0 +1,204 @@
+// -----------------------------------------------------------------------------
+// ProofGraphModels.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T1 - Proof graph visualization models
+// Description: Directed acyclic graph representation of the full evidence
+//              chain backing a verdict. Nodes represent evidence artifacts,
+//              edges represent derivation/dependency relationships, and
+//              paths show the full chain from source evidence to verdict.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Explainability;
+
+/// <summary>
+/// Complete directed acyclic graph representing the evidence chain
+/// from source artifacts to a final verdict decision.
+/// </summary>
+public sealed record ProofGraph
+{
+    /// <summary>Content-addressed graph identifier.</summary>
+    [JsonPropertyName("graph_id")]
+    public required string GraphId { get; init; }
+
+    /// <summary>Reference to the verdict this graph explains.</summary>
+    [JsonPropertyName("verdict_ref")]
+    public required VerdictReference VerdictRef { get; init; }
+
+    /// <summary>All nodes in the proof graph.</summary>
+    [JsonPropertyName("nodes")]
+    public required ImmutableArray<ProofGraphNode> Nodes { get; init; }
+
+    /// <summary>All edges in the proof graph.</summary>
+    [JsonPropertyName("edges")]
+    public required ImmutableArray<ProofGraphEdge> Edges { get; init; }
+
+    /// <summary>Critical paths from source evidence to verdict.</summary>
+    [JsonPropertyName("critical_paths")]
+    public required ImmutableArray<ProofGraphPath> CriticalPaths { get; init; }
+
+    /// <summary>Root node ID (the verdict node).</summary>
+    [JsonPropertyName("root_node_id")]
+    public required string RootNodeId { get; init; }
+
+    /// <summary>Leaf node IDs (source evidence).</summary>
+    [JsonPropertyName("leaf_node_ids")]
+    public required ImmutableArray<string> LeafNodeIds { get; init; }
+
+    /// <summary>When the graph was computed.</summary>
+    [JsonPropertyName("computed_at")]
+    public required DateTimeOffset ComputedAt { get; init; }
+}
+
+/// <summary>
+/// A node in the proof graph representing an evidence artifact,
+/// intermediate computation, or the final verdict.
+/// </summary>
+public sealed record ProofGraphNode
+{
+    /// <summary>Unique node identifier.</summary>
+    [JsonPropertyName("id")]
+    public required string Id { get; init; }
+
+    /// <summary>Human-readable label for display.</summary>
+    [JsonPropertyName("label")]
+    public required string Label { get; init; }
+
+    /// <summary>Node type classification.</summary>
+    [JsonPropertyName("type")]
+    public required ProofNodeType Type { get; init; }
+
+    /// <summary>Confidence score at this node (0.0 to 1.0).</summary>
+    [JsonPropertyName("confidence")]
+    public double? Confidence { get; init; }
+
+    /// <summary>Score contribution of this node to the verdict.</summary>
+    [JsonPropertyName("score_contribution")]
+    public double? ScoreContribution { get; init; }
+
+    /// <summary>Content digest of the underlying artifact.</summary>
+    [JsonPropertyName("digest")]
+    public string? Digest { get; init; }
+
+    /// <summary>Additional metadata for display.</summary>
+    [JsonPropertyName("metadata")]
+    public ImmutableDictionary<string, string> Metadata { get; init; } =
+        ImmutableDictionary<string, string>.Empty;
+
+    /// <summary>Visual depth in the graph (0 = verdict root).</summary>
+    [JsonPropertyName("depth")]
+    public int Depth { get; init; }
+}
+
+/// <summary>
+/// Classification of proof graph nodes.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter))]
+public enum ProofNodeType
+{
+    /// <summary>Final verdict decision.</summary>
+    Verdict,
+
+    /// <summary>Policy rule evaluation.</summary>
+    PolicyRule,
+
+    /// <summary>Scoring computation (e.g., EWS dimension).</summary>
+    ScoreComputation,
+
+    /// <summary>VEX statement evidence.</summary>
+    VexStatement,
+
+    /// <summary>Reachability analysis result.</summary>
+    ReachabilityAnalysis,
+
+    /// <summary>SBOM lineage evidence.</summary>
+    SbomEvidence,
+
+    /// <summary>Provenance attestation.</summary>
+    Provenance,
+
+    /// <summary>Runtime signal observation.</summary>
+    RuntimeSignal,
+
+    /// <summary>EPSS/CVSS advisory data.</summary>
+    AdvisoryData,
+
+    /// <summary>Guardrail rule application.</summary>
+    Guardrail,
+
+    /// <summary>Counterfactual hypothesis node.</summary>
+    Counterfactual
+}
+
+/// <summary>
+/// A directed edge in the proof graph showing derivation.
+/// </summary>
+public sealed record ProofGraphEdge
+{
+    /// <summary>Source node ID (evidence provider).</summary>
+    [JsonPropertyName("source")]
+    public required string Source { get; init; }
+
+    /// <summary>Target node ID (evidence consumer).</summary>
+    [JsonPropertyName("target")]
+    public required string Target { get; init; }
+
+    /// <summary>Relationship type.</summary>
+    [JsonPropertyName("relation")]
+    public required ProofEdgeRelation Relation { get; init; }
+
+    /// <summary>Weight/importance of this edge (0.0 to 1.0).</summary>
+    [JsonPropertyName("weight")]
+    public double Weight { get; init; } = 1.0;
+
+    /// <summary>Human-readable label for the edge.</summary>
+    [JsonPropertyName("label")]
+    public string? Label { get; init; }
+}
+
+/// <summary>
+/// Types of relationships between proof graph nodes.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter))]
+public enum ProofEdgeRelation
+{
+    /// <summary>Source provides input evidence to target.</summary>
+    ProvidesEvidence,
+
+    /// <summary>Source score contributes to target aggregate.</summary>
+    ContributesScore,
+
+    /// <summary>Source evaluation gates target decision.</summary>
+    Gates,
+
+    /// <summary>Source attestation supports target claim.</summary>
+    Attests,
+
+    /// <summary>Source overrides target under certain conditions.</summary>
+    Overrides,
+
+    /// <summary>Source guardrail modifies target score.</summary>
+    GuardrailApplied
+}
+
+/// <summary>
+/// A path through the proof graph from a leaf evidence node
+/// to the root verdict node.
+/// </summary>
+public sealed record ProofGraphPath
+{
+    /// <summary>Ordered node IDs from leaf to root.</summary>
+    [JsonPropertyName("node_ids")]
+    public required ImmutableArray<string> NodeIds { get; init; }
+
+    /// <summary>Cumulative confidence along this path.</summary>
+    [JsonPropertyName("path_confidence")]
+    public required double PathConfidence { get; init; }
+
+    /// <summary>Whether this path is the highest-confidence path.</summary>
+    [JsonPropertyName("is_critical")]
+    public bool IsCritical { get; init; }
+
+    /// <summary>Human-readable description of this evidence chain.</summary>
+    [JsonPropertyName("description")]
+    public required string Description { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Explainability/ProofStudioService.cs

@@ -0,0 +1,272 @@
+// -----------------------------------------------------------------------------
+// ProofStudioService.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T2 - Integration service wiring proof graph + score breakdown
+// Description: Orchestrates proof graph construction and score breakdown
+//              composition from existing policy engine data models.
+//              Bridges ScoreExplanation (Policy.Scoring) and VerdictRationale
+//              (Explainability) into the proof studio visualization models.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Diagnostics.Metrics;
+
+namespace StellaOps.Policy.Explainability;
+
+/// <summary>
+/// Integration surface for the Proof Studio UX.
+/// Composes proof graphs and score breakdowns from existing
+/// policy engine results.
+/// </summary>
+public interface IProofStudioService
+{
+    /// <summary>
+    /// Builds a full proof studio view from a verdict rationale and
+    /// optional per-factor score explanation data.
+    /// </summary>
+    ProofStudioView Compose(ProofStudioRequest request);
+
+    /// <summary>
+    /// Applies a counterfactual scenario to an existing proof studio view,
+    /// returning a new view with the overlay applied.
+    /// </summary>
+    ProofStudioView ApplyCounterfactual(
+        ProofStudioView current,
+        CounterfactualScenario scenario);
+}
+
+/// <summary>
+/// Request to compose a proof studio view.
+/// </summary>
+public sealed record ProofStudioRequest
+{
+    /// <summary>Verdict rationale from the explainability module.</summary>
+    public required VerdictRationale Rationale { get; init; }
+
+    /// <summary>Per-factor score explanations from scoring engine.</summary>
+    public IReadOnlyList<ScoreFactorInput>? ScoreFactors { get; init; }
+
+    /// <summary>Composite score (0-100).</summary>
+    public int? CompositeScore { get; init; }
+
+    /// <summary>Action bucket label.</summary>
+    public string? ActionBucket { get; init; }
+
+    /// <summary>Guardrail applications, if any.</summary>
+    public IReadOnlyList<GuardrailInput>? Guardrails { get; init; }
+
+    /// <summary>Entropy value (0-1).</summary>
+    public double? Entropy { get; init; }
+
+    /// <summary>Whether manual review is required.</summary>
+    public bool NeedsReview { get; init; }
+}
+
+/// <summary>
+/// Score factor input from the scoring engine.
+/// </summary>
+public sealed record ScoreFactorInput
+{
+    /// <summary>Factor identifier (e.g., "reachability", "evidence").</summary>
+    public required string Factor { get; init; }
+
+    /// <summary>Raw factor value (0-100).</summary>
+    public required int Value { get; init; }
+
+    /// <summary>Weight applied to this factor (0-1).</summary>
+    public double Weight { get; init; }
+
+    /// <summary>Confidence in this factor's accuracy (0-1).</summary>
+    public double Confidence { get; init; } = 1.0;
+
+    /// <summary>Human-readable explanation.</summary>
+    public required string Reason { get; init; }
+
+    /// <summary>Whether this factor is subtractive.</summary>
+    public bool IsSubtractive { get; init; }
+
+    /// <summary>Contributing evidence digests.</summary>
+    public IReadOnlyList<string>? ContributingDigests { get; init; }
+}
+
+/// <summary>
+/// Guardrail application input from the scoring engine.
+/// </summary>
+public sealed record GuardrailInput
+{
+    public required string Name { get; init; }
+    public int ScoreBefore { get; init; }
+    public int ScoreAfter { get; init; }
+    public required string Reason { get; init; }
+    public IReadOnlyList<string>? Conditions { get; init; }
+}
+
+/// <summary>
+/// Complete proof studio view combining graph and dashboard.
+/// </summary>
+public sealed record ProofStudioView
+{
+    /// <summary>The proof graph DAG.</summary>
+    [JsonPropertyName("proof_graph")]
+    public required ProofGraph ProofGraph { get; init; }
+
+    /// <summary>The score breakdown dashboard.</summary>
+    [JsonPropertyName("score_breakdown")]
+    public ScoreBreakdownDashboard? ScoreBreakdown { get; init; }
+
+    /// <summary>When this view was composed.</summary>
+    [JsonPropertyName("composed_at")]
+    public required DateTimeOffset ComposedAt { get; init; }
+}
+
+/// <summary>
+/// Default implementation of <see cref="IProofStudioService"/>.
+/// </summary>
+public sealed class ProofStudioService : IProofStudioService
+{
+    private readonly IProofGraphBuilder _graphBuilder;
+    private readonly ILogger<ProofStudioService> _logger;
+    private readonly Counter<long> _viewsComposed;
+    private readonly Counter<long> _counterfactualsApplied;
+
+    public ProofStudioService(
+        IProofGraphBuilder graphBuilder,
+        ILogger<ProofStudioService> logger,
+        IMeterFactory meterFactory)
+    {
+        _graphBuilder = graphBuilder;
+        _logger = logger;
+
+        var meter = meterFactory.Create("StellaOps.Policy.Explainability.ProofStudio");
+        _viewsComposed = meter.CreateCounter<long>(
+            "stellaops.proofstudio.views_composed_total",
+            description: "Total proof studio views composed");
+        _counterfactualsApplied = meter.CreateCounter<long>(
+            "stellaops.proofstudio.counterfactuals_applied_total",
+            description: "Total counterfactual scenarios applied");
+    }
+
+    public ProofStudioView Compose(ProofStudioRequest request)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        var now = DateTimeOffset.UtcNow;
+
+        // Build score breakdown dashboard if factor data is available
+        ScoreBreakdownDashboard? dashboard = null;
+        if (request.ScoreFactors is { Count: > 0 })
+        {
+            dashboard = BuildDashboard(request, now);
+        }
+
+        // Build proof graph
+        var graphInput = new ProofGraphInput
+        {
+            Rationale = request.Rationale,
+            ScoreBreakdown = dashboard,
+            ComputedAt = now
+        };
+
+        var proofGraph = _graphBuilder.Build(graphInput);
+
+        _viewsComposed.Add(1);
+        _logger.LogDebug(
+            "Composed proof studio view {GraphId} with {HasDashboard} dashboard",
+            proofGraph.GraphId, dashboard is not null);
+
+        return new ProofStudioView
+        {
+            ProofGraph = proofGraph,
+            ScoreBreakdown = dashboard,
+            ComposedAt = now
+        };
+    }
+
+    public ProofStudioView ApplyCounterfactual(
+        ProofStudioView current,
+        CounterfactualScenario scenario)
+    {
+        ArgumentNullException.ThrowIfNull(current);
+        ArgumentNullException.ThrowIfNull(scenario);
+
+        var overlayGraph = _graphBuilder.AddCounterfactualOverlay(
+            current.ProofGraph, scenario);
+
+        _counterfactualsApplied.Add(1);
+        _logger.LogDebug(
+            "Applied counterfactual '{Label}' to graph {GraphId}",
+            scenario.Label, current.ProofGraph.GraphId);
+
+        return current with
+        {
+            ProofGraph = overlayGraph,
+            ComposedAt = DateTimeOffset.UtcNow
+        };
+    }
+
+    // ── Private helpers ──────────────────────────────────────────────────
+
+    private static ScoreBreakdownDashboard BuildDashboard(
+        ProofStudioRequest request,
+        DateTimeOffset computedAt)
+    {
+        var factors = request.ScoreFactors!
+            .Select(f => new FactorContribution
+            {
+                FactorId = f.Factor,
+                FactorName = FormatFactorName(f.Factor),
+                RawScore = f.Value,
+                Weight = f.Weight,
+                Confidence = f.Confidence,
+                IsSubtractive = f.IsSubtractive,
+                EvidenceSource = f.ContributingDigests?.FirstOrDefault(),
+                Explanation = f.Reason
+            })
+            .ToImmutableArray();
+
+        var guardrails = (request.Guardrails ?? [])
+            .Select(g => new GuardrailApplication
+            {
+                GuardrailName = g.Name,
+                ScoreBefore = g.ScoreBefore,
+                ScoreAfter = g.ScoreAfter,
+                Reason = g.Reason,
+                Conditions = g.Conditions is not null
+                    ? [.. g.Conditions]
+                    : []
+            })
+            .ToImmutableArray();
+
+        return new ScoreBreakdownDashboard
+        {
+            DashboardId = $"dash:{Guid.CreateVersion7():N}",
+            VerdictRef = request.Rationale.VerdictRef,
+            CompositeScore = request.CompositeScore ?? 0,
+            ActionBucket = request.ActionBucket ?? "Unknown",
+            Factors = factors,
+            GuardrailsApplied = guardrails,
+            PreGuardrailScore = request.CompositeScore ?? 0,
+            Entropy = request.Entropy ?? 0.0,
+            NeedsReview = request.NeedsReview,
+            ComputedAt = computedAt
+        };
+    }
+
+    private static string FormatFactorName(string factorId)
+    {
+        return factorId switch
+        {
+            "reachability" or "rch" => "Reachability",
+            "evidence" or "evd" => "Evidence",
+            "provenance" or "prv" => "Provenance",
+            "baseSeverity" or "sev" => "Base Severity",
+            "runtimeSignal" or "rts" => "Runtime Signal",
+            "mitigation" or "mit" => "Mitigation",
+            "exploit" or "exp" => "Exploit Maturity",
+            "temporal" or "tmp" => "Temporal",
+            _ => factorId.Length > 0
+                ? char.ToUpperInvariant(factorId[0]) + factorId[1..]
+                : factorId
+        };
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Explainability/ScoreBreakdownDashboard.cs

@@ -0,0 +1,131 @@
+// -----------------------------------------------------------------------------
+// ScoreBreakdownDashboard.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T1 - Score breakdown dashboard data models
+// Description: Per-factor score breakdown for dashboard visualization.
+//              Produces chart-ready data showing how each scoring dimension
+//              contributes to the final verdict score.
+// -----------------------------------------------------------------------------
+
+namespace StellaOps.Policy.Explainability;
+
+/// <summary>
+/// Complete score breakdown for dashboard visualization, showing
+/// per-factor contributions to the final verdict score.
+/// </summary>
+public sealed record ScoreBreakdownDashboard
+{
+    /// <summary>Content-addressed dashboard identifier.</summary>
+    [JsonPropertyName("dashboard_id")]
+    public required string DashboardId { get; init; }
+
+    /// <summary>Reference to the verdict being broken down.</summary>
+    [JsonPropertyName("verdict_ref")]
+    public required VerdictReference VerdictRef { get; init; }
+
+    /// <summary>Overall composite score (0-100).</summary>
+    [JsonPropertyName("composite_score")]
+    public required int CompositeScore { get; init; }
+
+    /// <summary>Action bucket label (e.g., "Act Now", "Schedule Next").</summary>
+    [JsonPropertyName("action_bucket")]
+    public required string ActionBucket { get; init; }
+
+    /// <summary>Per-factor contribution breakdown for chart rendering.</summary>
+    [JsonPropertyName("factors")]
+    public required ImmutableArray<FactorContribution> Factors { get; init; }
+
+    /// <summary>Guardrails that were applied, if any.</summary>
+    [JsonPropertyName("guardrails_applied")]
+    public ImmutableArray<GuardrailApplication> GuardrailsApplied { get; init; } = [];
+
+    /// <summary>Score before guardrails were applied.</summary>
+    [JsonPropertyName("pre_guardrail_score")]
+    public int? PreGuardrailScore { get; init; }
+
+    /// <summary>Entropy level for determinization decisions.</summary>
+    [JsonPropertyName("entropy")]
+    public double? Entropy { get; init; }
+
+    /// <summary>Whether this verdict needs manual review based on entropy.</summary>
+    [JsonPropertyName("needs_review")]
+    public bool NeedsReview { get; init; }
+
+    /// <summary>When the breakdown was computed.</summary>
+    [JsonPropertyName("computed_at")]
+    public required DateTimeOffset ComputedAt { get; init; }
+}
+
+/// <summary>
+/// Individual factor contribution to the composite score.
+/// </summary>
+public sealed record FactorContribution
+{
+    /// <summary>Factor identifier (e.g., "rch", "rts", "bkp").</summary>
+    [JsonPropertyName("factor_id")]
+    public required string FactorId { get; init; }
+
+    /// <summary>Human-readable factor name.</summary>
+    [JsonPropertyName("factor_name")]
+    public required string FactorName { get; init; }
+
+    /// <summary>Raw normalized score for this factor (0-100).</summary>
+    [JsonPropertyName("raw_score")]
+    public required int RawScore { get; init; }
+
+    /// <summary>Weight assigned to this factor (0.0-1.0).</summary>
+    [JsonPropertyName("weight")]
+    public required double Weight { get; init; }
+
+    /// <summary>Weighted contribution to composite score.</summary>
+    [JsonPropertyName("weighted_contribution")]
+    public double WeightedContribution => RawScore * Weight;
+
+    /// <summary>Confidence level for this factor (0.0-1.0).</summary>
+    [JsonPropertyName("confidence")]
+    public required double Confidence { get; init; }
+
+    /// <summary>
+    /// Whether this is a subtractive factor (reduces risk).
+    /// </summary>
+    [JsonPropertyName("is_subtractive")]
+    public bool IsSubtractive { get; init; }
+
+    /// <summary>Source of the evidence for this factor.</summary>
+    [JsonPropertyName("evidence_source")]
+    public string? EvidenceSource { get; init; }
+
+    /// <summary>Human-readable explanation of the score.</summary>
+    [JsonPropertyName("explanation")]
+    public required string Explanation { get; init; }
+
+    /// <summary>Percentage of composite that this factor contributes.</summary>
+    [JsonPropertyName("percentage_of_total")]
+    public double PercentageOfTotal { get; init; }
+}
+
+/// <summary>
+/// Record of a guardrail being applied to the score.
+/// </summary>
+public sealed record GuardrailApplication
+{
+    /// <summary>Guardrail name (e.g., "notAffectedCap", "runtimeFloor").</summary>
+    [JsonPropertyName("guardrail_name")]
+    public required string GuardrailName { get; init; }
+
+    /// <summary>Score before this guardrail.</summary>
+    [JsonPropertyName("score_before")]
+    public required int ScoreBefore { get; init; }
+
+    /// <summary>Score after this guardrail.</summary>
+    [JsonPropertyName("score_after")]
+    public required int ScoreAfter { get; init; }
+
+    /// <summary>Human-readable reason the guardrail triggered.</summary>
+    [JsonPropertyName("reason")]
+    public required string Reason { get; init; }
+
+    /// <summary>Conditions that caused the guardrail to fire.</summary>
+    [JsonPropertyName("conditions")]
+    public ImmutableArray<string> Conditions { get; init; } = [];
+}

src/Policy/__Libraries/StellaOps.Policy.Explainability/ServiceCollectionExtensions.cs

@@ -7,6 +7,8 @@ public static class ExplainabilityServiceCollectionExtensions
     public static IServiceCollection AddVerdictExplainability(this IServiceCollection services)
     {
         services.AddSingleton<IVerdictRationaleRenderer, VerdictRationaleRenderer>();
+        services.AddSingleton<IProofGraphBuilder, ProofGraphBuilder>();
+        services.AddSingleton<IProofStudioService, ProofStudioService>();
         return services;
     }
 }

src/Policy/__Libraries/StellaOps.Policy.Interop/Abstractions/IPolicyDiffMerge.cs

@@ -0,0 +1,153 @@
+// -----------------------------------------------------------------------------
+// IPolicyDiffMerge.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - Policy diff/merge interface
+// Description: Interface for diffing and merging PolicyPackDocuments.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Interop.Contracts;
+
+namespace StellaOps.Policy.Interop.Abstractions;
+
+/// <summary>
+/// Computes structural diffs between two PolicyPackDocuments and merges packs.
+/// </summary>
+public interface IPolicyDiffMerge
+{
+    /// <summary>
+    /// Computes a structural diff between two policy pack documents.
+    /// Returns a list of changes (additions, removals, modifications).
+    /// </summary>
+    PolicyDiffResult Diff(PolicyPackDocument baseline, PolicyPackDocument updated);
+
+    /// <summary>
+    /// Merges two policy pack documents according to the specified strategy.
+    /// </summary>
+    PolicyMergeResult Merge(
+        PolicyPackDocument baseDoc,
+        PolicyPackDocument overlay,
+        PolicyMergeStrategy strategy = PolicyMergeStrategy.OverlayWins);
+}
+
+/// <summary>
+/// Result of a policy diff operation.
+/// </summary>
+public sealed record PolicyDiffResult
+{
+    /// <summary>Whether the two documents are identical.</summary>
+    public required bool AreIdentical { get; init; }
+
+    /// <summary>Ordered list of changes between baseline and updated.</summary>
+    public IReadOnlyList<PolicyChange> Changes { get; init; } = [];
+
+    /// <summary>Summary statistics about the diff.</summary>
+    public required PolicyDiffSummary Summary { get; init; }
+}
+
+/// <summary>
+/// A single change between two policy documents.
+/// </summary>
+public sealed record PolicyChange
+{
+    /// <summary>Type of change.</summary>
+    public required PolicyChangeType ChangeType { get; init; }
+
+    /// <summary>Category: "gate", "rule", "setting", "metadata".</summary>
+    public required string Category { get; init; }
+
+    /// <summary>Path to the changed element (e.g., "gates[cvss-threshold].config.threshold").</summary>
+    public required string Path { get; init; }
+
+    /// <summary>Previous value (null for additions).</summary>
+    public object? OldValue { get; init; }
+
+    /// <summary>New value (null for removals).</summary>
+    public object? NewValue { get; init; }
+
+    /// <summary>Human-readable description of the change.</summary>
+    public required string Description { get; init; }
+}
+
+/// <summary>
+/// Type of policy change.
+/// </summary>
+public enum PolicyChangeType
+{
+    /// <summary>Element was added in the updated document.</summary>
+    Added,
+
+    /// <summary>Element was removed in the updated document.</summary>
+    Removed,
+
+    /// <summary>Element was modified between documents.</summary>
+    Modified
+}
+
+/// <summary>
+/// Summary statistics for a policy diff.
+/// </summary>
+public sealed record PolicyDiffSummary
+{
+    /// <summary>Number of additions.</summary>
+    public int Additions { get; init; }
+
+    /// <summary>Number of removals.</summary>
+    public int Removals { get; init; }
+
+    /// <summary>Number of modifications.</summary>
+    public int Modifications { get; init; }
+
+    /// <summary>Total number of changes.</summary>
+    public int Total => Additions + Removals + Modifications;
+}
+
+/// <summary>
+/// Strategy for merging two policy pack documents.
+/// </summary>
+public enum PolicyMergeStrategy
+{
+    /// <summary>Overlay values win on conflict.</summary>
+    OverlayWins,
+
+    /// <summary>Base values win on conflict.</summary>
+    BaseWins,
+
+    /// <summary>Fail on any conflict.</summary>
+    FailOnConflict
+}
+
+/// <summary>
+/// Result of a policy merge operation.
+/// </summary>
+public sealed record PolicyMergeResult
+{
+    /// <summary>Whether the merge succeeded.</summary>
+    public required bool Success { get; init; }
+
+    /// <summary>Merged document (null if failed).</summary>
+    public PolicyPackDocument? Document { get; init; }
+
+    /// <summary>Conflicts encountered during merge (empty if OverlayWins/BaseWins).</summary>
+    public IReadOnlyList<PolicyMergeConflict> Conflicts { get; init; } = [];
+
+    /// <summary>Diagnostics from the merge operation.</summary>
+    public IReadOnlyList<PolicyDiagnostic> Diagnostics { get; init; } = [];
+}
+
+/// <summary>
+/// A conflict encountered during policy merge.
+/// </summary>
+public sealed record PolicyMergeConflict
+{
+    /// <summary>Path to the conflicting element.</summary>
+    public required string Path { get; init; }
+
+    /// <summary>Value from the base document.</summary>
+    public object? BaseValue { get; init; }
+
+    /// <summary>Value from the overlay document.</summary>
+    public object? OverlayValue { get; init; }
+
+    /// <summary>Human-readable description of the conflict.</summary>
+    public required string Description { get; init; }
+}

src/Policy/__Libraries/StellaOps.Policy.Interop/Abstractions/IPolicyYamlExporter.cs

@@ -0,0 +1,43 @@
+// -----------------------------------------------------------------------------
+// IPolicyYamlExporter.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - YAML export interface
+// Description: Interface for YAML export of PolicyPackDocuments.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Interop.Contracts;
+
+namespace StellaOps.Policy.Interop.Abstractions;
+
+/// <summary>
+/// Exports native C# policy packs to YAML format.
+/// </summary>
+public interface IPolicyYamlExporter
+{
+    /// <summary>
+    /// Exports the given policy pack document to canonical YAML format.
+    /// The output is deterministic: same input produces byte-identical output.
+    /// </summary>
+    Task<YamlExportResult> ExportToYamlAsync(
+        PolicyPackDocument document,
+        PolicyExportRequest request,
+        CancellationToken ct = default);
+}
+
+/// <summary>
+/// Result of a YAML export operation.
+/// </summary>
+public sealed record YamlExportResult
+{
+    /// <summary>Whether export succeeded.</summary>
+    public required bool Success { get; init; }
+
+    /// <summary>Generated YAML content.</summary>
+    public required string YamlContent { get; init; }
+
+    /// <summary>SHA-256 digest of the generated YAML.</summary>
+    public string? Digest { get; init; }
+
+    /// <summary>Warnings generated during export.</summary>
+    public IReadOnlyList<string> Warnings { get; init; } = [];
+}

src/Policy/__Libraries/StellaOps.Policy.Interop/Contracts/PolicyInteropModels.cs

@@ -320,12 +320,14 @@ public static class PolicyFormats
 {
     public const string Json = "json";
     public const string Rego = "rego";
+    public const string Yaml = "yaml";
 
-    public static readonly IReadOnlyList<string> All = [Json, Rego];
+    public static readonly IReadOnlyList<string> All = [Json, Rego, Yaml];
 
     public static bool IsValid(string format) =>
         string.Equals(format, Json, StringComparison.OrdinalIgnoreCase) ||
-        string.Equals(format, Rego, StringComparison.OrdinalIgnoreCase);
+        string.Equals(format, Rego, StringComparison.OrdinalIgnoreCase) ||
+        string.Equals(format, Yaml, StringComparison.OrdinalIgnoreCase);
 }
 
 /// <summary>

src/Policy/__Libraries/StellaOps.Policy.Interop/DependencyInjection/PolicyInteropServiceCollectionExtensions.cs

@@ -1,5 +1,9 @@
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using StellaOps.Policy.Interop.Abstractions;
+using StellaOps.Policy.Interop.DiffMerge;
+using StellaOps.Policy.Interop.Export;
+using StellaOps.Policy.Interop.Import;
 
 namespace StellaOps.Policy.Interop.DependencyInjection;
 
@@ -10,13 +14,26 @@ public static class PolicyInteropServiceCollectionExtensions
 {
     /// <summary>
     /// Adds Policy Interop services to the service collection.
-    /// Registers: IPolicyExporter, IPolicyImporter, IPolicyValidator,
+    /// Registers: IPolicyExporter, IPolicyImporter (JSON + YAML),
+    /// IPolicyYamlExporter, IPolicyDiffMerge, IPolicyValidator,
     /// IPolicyEvaluator, IRegoCodeGenerator, IRemediationResolver.
     /// </summary>
     public static IServiceCollection AddPolicyInterop(this IServiceCollection services)
     {
-        // Implementations are registered in TASK-02..05 when created.
-        // This extension point ensures consistent DI wiring.
+        // JSON export/import
+        services.TryAddSingleton<IPolicyExporter, JsonPolicyExporter>();
+        services.TryAddSingleton<JsonPolicyImporter>();
+
+        // YAML export/import
+        services.TryAddSingleton<IPolicyYamlExporter, YamlPolicyExporter>();
+        services.TryAddSingleton<YamlPolicyImporter>();
+
+        // Register both importers as IPolicyImporter (JSON is the primary/default)
+        services.TryAddSingleton<IPolicyImporter, JsonPolicyImporter>();
+
+        // Policy diff/merge engine
+        services.TryAddSingleton<IPolicyDiffMerge, PolicyDiffMergeEngine>();
+
         return services;
     }
 

src/Policy/__Libraries/StellaOps.Policy.Interop/DiffMerge/PolicyDiffMergeEngine.cs

@@ -0,0 +1,639 @@
+// -----------------------------------------------------------------------------
+// PolicyDiffMergeEngine.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - Policy diff/merge implementation
+// Description: Structural diff and merge engine for PolicyPackDocuments.
+// -----------------------------------------------------------------------------
+
+using System.Text.Json;
+using StellaOps.Policy.Interop.Abstractions;
+using StellaOps.Policy.Interop.Contracts;
+
+namespace StellaOps.Policy.Interop.DiffMerge;
+
+/// <summary>
+/// Computes structural diffs and merges for PolicyPackDocuments.
+/// All operations are deterministic and offline-safe.
+/// </summary>
+public sealed class PolicyDiffMergeEngine : IPolicyDiffMerge
+{
+    /// <inheritdoc/>
+    public PolicyDiffResult Diff(PolicyPackDocument baseline, PolicyPackDocument updated)
+    {
+        var changes = new List<PolicyChange>();
+
+        // Diff metadata
+        DiffMetadata(baseline.Metadata, updated.Metadata, changes);
+
+        // Diff settings
+        DiffSettings(baseline.Spec.Settings, updated.Spec.Settings, changes);
+
+        // Diff gates
+        DiffGates(baseline.Spec.Gates, updated.Spec.Gates, changes);
+
+        // Diff rules
+        DiffRules(baseline.Spec.Rules, updated.Spec.Rules, changes);
+
+        var summary = new PolicyDiffSummary
+        {
+            Additions = changes.Count(c => c.ChangeType == PolicyChangeType.Added),
+            Removals = changes.Count(c => c.ChangeType == PolicyChangeType.Removed),
+            Modifications = changes.Count(c => c.ChangeType == PolicyChangeType.Modified)
+        };
+
+        return new PolicyDiffResult
+        {
+            AreIdentical = changes.Count == 0,
+            Changes = changes,
+            Summary = summary
+        };
+    }
+
+    /// <inheritdoc/>
+    public PolicyMergeResult Merge(
+        PolicyPackDocument baseDoc,
+        PolicyPackDocument overlay,
+        PolicyMergeStrategy strategy = PolicyMergeStrategy.OverlayWins)
+    {
+        var conflicts = new List<PolicyMergeConflict>();
+        var diagnostics = new List<PolicyDiagnostic>();
+
+        // Merge metadata (overlay wins for description, version)
+        var mergedMeta = MergeMetadata(baseDoc.Metadata, overlay.Metadata, strategy, conflicts);
+
+        // Merge settings
+        var mergedSettings = MergeSettings(baseDoc.Spec.Settings, overlay.Spec.Settings, strategy, conflicts);
+
+        // Merge gates
+        var mergedGates = MergeGates(baseDoc.Spec.Gates, overlay.Spec.Gates, strategy, conflicts);
+
+        // Merge rules
+        var mergedRules = MergeRules(baseDoc.Spec.Rules, overlay.Spec.Rules, strategy, conflicts);
+
+        // Fail on conflicts if strategy demands it
+        if (strategy == PolicyMergeStrategy.FailOnConflict && conflicts.Count > 0)
+        {
+            diagnostics.Add(new PolicyDiagnostic
+            {
+                Severity = PolicyDiagnostic.Severities.Error,
+                Code = "MERGE_CONFLICT",
+                Message = $"Merge failed: {conflicts.Count} conflict(s) found."
+            });
+
+            return new PolicyMergeResult
+            {
+                Success = false,
+                Document = null,
+                Conflicts = conflicts,
+                Diagnostics = diagnostics
+            };
+        }
+
+        var merged = new PolicyPackDocument
+        {
+            ApiVersion = baseDoc.ApiVersion,
+            Kind = baseDoc.Kind,
+            Metadata = mergedMeta,
+            Spec = new PolicyPackSpec
+            {
+                Settings = mergedSettings,
+                Gates = mergedGates,
+                Rules = mergedRules
+            }
+        };
+
+        if (conflicts.Count > 0)
+        {
+            diagnostics.Add(new PolicyDiagnostic
+            {
+                Severity = PolicyDiagnostic.Severities.Warning,
+                Code = "MERGE_CONFLICTS_RESOLVED",
+                Message = $"{conflicts.Count} conflict(s) resolved using {strategy} strategy."
+            });
+        }
+
+        return new PolicyMergeResult
+        {
+            Success = true,
+            Document = merged,
+            Conflicts = conflicts,
+            Diagnostics = diagnostics
+        };
+    }
+
+    #region Diff Methods
+
+    private static void DiffMetadata(
+        PolicyPackMetadata baseline, PolicyPackMetadata updated, List<PolicyChange> changes)
+    {
+        if (baseline.Name != updated.Name)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "metadata",
+                Path = "metadata.name",
+                OldValue = baseline.Name,
+                NewValue = updated.Name,
+                Description = $"Name changed from '{baseline.Name}' to '{updated.Name}'."
+            });
+        }
+
+        if (baseline.Version != updated.Version)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "metadata",
+                Path = "metadata.version",
+                OldValue = baseline.Version,
+                NewValue = updated.Version,
+                Description = $"Version changed from '{baseline.Version}' to '{updated.Version}'."
+            });
+        }
+
+        if (baseline.Description != updated.Description)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "metadata",
+                Path = "metadata.description",
+                OldValue = baseline.Description,
+                NewValue = updated.Description,
+                Description = "Description changed."
+            });
+        }
+    }
+
+    private static void DiffSettings(
+        PolicyPackSettings baseline, PolicyPackSettings updated, List<PolicyChange> changes)
+    {
+        if (baseline.DefaultAction != updated.DefaultAction)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "setting",
+                Path = "spec.settings.defaultAction",
+                OldValue = baseline.DefaultAction,
+                NewValue = updated.DefaultAction,
+                Description = $"Default action changed from '{baseline.DefaultAction}' to '{updated.DefaultAction}'."
+            });
+        }
+
+        if (Math.Abs(baseline.UnknownsThreshold - updated.UnknownsThreshold) > 1e-10)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "setting",
+                Path = "spec.settings.unknownsThreshold",
+                OldValue = baseline.UnknownsThreshold,
+                NewValue = updated.UnknownsThreshold,
+                Description = $"Unknowns threshold changed from {baseline.UnknownsThreshold} to {updated.UnknownsThreshold}."
+            });
+        }
+
+        if (baseline.StopOnFirstFailure != updated.StopOnFirstFailure)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "setting",
+                Path = "spec.settings.stopOnFirstFailure",
+                OldValue = baseline.StopOnFirstFailure,
+                NewValue = updated.StopOnFirstFailure,
+                Description = $"StopOnFirstFailure changed from {baseline.StopOnFirstFailure} to {updated.StopOnFirstFailure}."
+            });
+        }
+
+        if (baseline.DeterministicMode != updated.DeterministicMode)
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Modified,
+                Category = "setting",
+                Path = "spec.settings.deterministicMode",
+                OldValue = baseline.DeterministicMode,
+                NewValue = updated.DeterministicMode,
+                Description = $"DeterministicMode changed from {baseline.DeterministicMode} to {updated.DeterministicMode}."
+            });
+        }
+    }
+
+    private static void DiffGates(
+        IReadOnlyList<PolicyGateDefinition> baselineGates,
+        IReadOnlyList<PolicyGateDefinition> updatedGates,
+        List<PolicyChange> changes)
+    {
+        var baseMap = baselineGates.ToDictionary(g => g.Id);
+        var updatedMap = updatedGates.ToDictionary(g => g.Id);
+
+        // Removed gates
+        foreach (var id in baseMap.Keys.Except(updatedMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Removed,
+                Category = "gate",
+                Path = $"spec.gates[{id}]",
+                OldValue = baseMap[id].Type,
+                NewValue = null,
+                Description = $"Gate '{id}' ({baseMap[id].Type}) removed."
+            });
+        }
+
+        // Added gates
+        foreach (var id in updatedMap.Keys.Except(baseMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Added,
+                Category = "gate",
+                Path = $"spec.gates[{id}]",
+                OldValue = null,
+                NewValue = updatedMap[id].Type,
+                Description = $"Gate '{id}' ({updatedMap[id].Type}) added."
+            });
+        }
+
+        // Modified gates
+        foreach (var id in baseMap.Keys.Intersect(updatedMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            var baseGate = baseMap[id];
+            var updatedGate = updatedMap[id];
+
+            if (baseGate.Enabled != updatedGate.Enabled)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Modified,
+                    Category = "gate",
+                    Path = $"spec.gates[{id}].enabled",
+                    OldValue = baseGate.Enabled,
+                    NewValue = updatedGate.Enabled,
+                    Description = $"Gate '{id}' enabled changed from {baseGate.Enabled} to {updatedGate.Enabled}."
+                });
+            }
+
+            if (baseGate.Type != updatedGate.Type)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Modified,
+                    Category = "gate",
+                    Path = $"spec.gates[{id}].type",
+                    OldValue = baseGate.Type,
+                    NewValue = updatedGate.Type,
+                    Description = $"Gate '{id}' type changed from '{baseGate.Type}' to '{updatedGate.Type}'."
+                });
+            }
+
+            // Diff config values
+            DiffDictionary(baseGate.Config, updatedGate.Config, $"spec.gates[{id}].config", "gate", changes);
+        }
+    }
+
+    private static void DiffRules(
+        IReadOnlyList<PolicyRuleDefinition> baselineRules,
+        IReadOnlyList<PolicyRuleDefinition> updatedRules,
+        List<PolicyChange> changes)
+    {
+        var baseMap = baselineRules.ToDictionary(r => r.Name);
+        var updatedMap = updatedRules.ToDictionary(r => r.Name);
+
+        // Removed rules
+        foreach (var name in baseMap.Keys.Except(updatedMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Removed,
+                Category = "rule",
+                Path = $"spec.rules[{name}]",
+                OldValue = baseMap[name].Action,
+                NewValue = null,
+                Description = $"Rule '{name}' removed."
+            });
+        }
+
+        // Added rules
+        foreach (var name in updatedMap.Keys.Except(baseMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            changes.Add(new PolicyChange
+            {
+                ChangeType = PolicyChangeType.Added,
+                Category = "rule",
+                Path = $"spec.rules[{name}]",
+                OldValue = null,
+                NewValue = updatedMap[name].Action,
+                Description = $"Rule '{name}' added."
+            });
+        }
+
+        // Modified rules
+        foreach (var name in baseMap.Keys.Intersect(updatedMap.Keys).OrderBy(k => k, StringComparer.Ordinal))
+        {
+            var baseRule = baseMap[name];
+            var updatedRule = updatedMap[name];
+
+            if (baseRule.Action != updatedRule.Action)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Modified,
+                    Category = "rule",
+                    Path = $"spec.rules[{name}].action",
+                    OldValue = baseRule.Action,
+                    NewValue = updatedRule.Action,
+                    Description = $"Rule '{name}' action changed from '{baseRule.Action}' to '{updatedRule.Action}'."
+                });
+            }
+
+            if (baseRule.Priority != updatedRule.Priority)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Modified,
+                    Category = "rule",
+                    Path = $"spec.rules[{name}].priority",
+                    OldValue = baseRule.Priority,
+                    NewValue = updatedRule.Priority,
+                    Description = $"Rule '{name}' priority changed from {baseRule.Priority} to {updatedRule.Priority}."
+                });
+            }
+
+            DiffDictionary(baseRule.Match, updatedRule.Match, $"spec.rules[{name}].match", "rule", changes);
+        }
+    }
+
+    private static void DiffDictionary(
+        IReadOnlyDictionary<string, object?> baseDict,
+        IReadOnlyDictionary<string, object?> updatedDict,
+        string pathPrefix,
+        string category,
+        List<PolicyChange> changes)
+    {
+        var allKeys = baseDict.Keys.Union(updatedDict.Keys).OrderBy(k => k, StringComparer.Ordinal);
+
+        foreach (var key in allKeys)
+        {
+            var hasBase = baseDict.TryGetValue(key, out var baseVal);
+            var hasUpdated = updatedDict.TryGetValue(key, out var updatedVal);
+
+            if (hasBase && !hasUpdated)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Removed,
+                    Category = category,
+                    Path = $"{pathPrefix}.{key}",
+                    OldValue = baseVal,
+                    NewValue = null,
+                    Description = $"Config key '{key}' removed."
+                });
+            }
+            else if (!hasBase && hasUpdated)
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Added,
+                    Category = category,
+                    Path = $"{pathPrefix}.{key}",
+                    OldValue = null,
+                    NewValue = updatedVal,
+                    Description = $"Config key '{key}' added with value '{updatedVal}'."
+                });
+            }
+            else if (hasBase && hasUpdated && !ValuesEqual(baseVal, updatedVal))
+            {
+                changes.Add(new PolicyChange
+                {
+                    ChangeType = PolicyChangeType.Modified,
+                    Category = category,
+                    Path = $"{pathPrefix}.{key}",
+                    OldValue = baseVal,
+                    NewValue = updatedVal,
+                    Description = $"Config key '{key}' changed from '{baseVal}' to '{updatedVal}'."
+                });
+            }
+        }
+    }
+
+    private static bool ValuesEqual(object? a, object? b)
+    {
+        if (a is null && b is null) return true;
+        if (a is null || b is null) return false;
+
+        // Handle JsonElement comparison (from System.Text.Json deserialization)
+        if (a is JsonElement ja && b is JsonElement jb)
+            return ja.GetRawText() == jb.GetRawText();
+
+        return a.Equals(b);
+    }
+
+    #endregion
+
+    #region Merge Methods
+
+    private static PolicyPackMetadata MergeMetadata(
+        PolicyPackMetadata baseM,
+        PolicyPackMetadata overlay,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        var name = ResolveConflict(baseM.Name, overlay.Name, "metadata.name", strategy, conflicts);
+        var version = ResolveConflict(baseM.Version, overlay.Version, "metadata.version", strategy, conflicts);
+        var description = overlay.Description ?? baseM.Description;
+
+        return new PolicyPackMetadata
+        {
+            Name = name ?? baseM.Name,
+            Version = version ?? baseM.Version,
+            Description = description,
+            Digest = null, // Digest will be recomputed on export
+            CreatedAt = baseM.CreatedAt,
+            ExportedFrom = baseM.ExportedFrom,
+            Parent = overlay.Parent ?? baseM.Parent,
+            Environment = overlay.Environment ?? baseM.Environment
+        };
+    }
+
+    private static PolicyPackSettings MergeSettings(
+        PolicyPackSettings baseS,
+        PolicyPackSettings overlay,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        var defaultAction = ResolveConflict(
+            baseS.DefaultAction, overlay.DefaultAction,
+            "spec.settings.defaultAction", strategy, conflicts) ?? baseS.DefaultAction;
+
+        return new PolicyPackSettings
+        {
+            DefaultAction = defaultAction,
+            UnknownsThreshold = strategy == PolicyMergeStrategy.BaseWins
+                ? baseS.UnknownsThreshold
+                : overlay.UnknownsThreshold,
+            StopOnFirstFailure = strategy == PolicyMergeStrategy.BaseWins
+                ? baseS.StopOnFirstFailure
+                : overlay.StopOnFirstFailure,
+            DeterministicMode = strategy == PolicyMergeStrategy.BaseWins
+                ? baseS.DeterministicMode
+                : overlay.DeterministicMode
+        };
+    }
+
+    private static List<PolicyGateDefinition> MergeGates(
+        IReadOnlyList<PolicyGateDefinition> baseGates,
+        IReadOnlyList<PolicyGateDefinition> overlayGates,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        var baseMap = baseGates.ToDictionary(g => g.Id);
+        var overlayMap = overlayGates.ToDictionary(g => g.Id);
+        var result = new List<PolicyGateDefinition>();
+
+        // Include all base gates (potentially overridden)
+        foreach (var gate in baseGates)
+        {
+            if (overlayMap.TryGetValue(gate.Id, out var overlayGate))
+            {
+                // Gate exists in both: merge configs
+                var mergedConfig = MergeDictionaries(
+                    gate.Config, overlayGate.Config,
+                    $"spec.gates[{gate.Id}].config", strategy, conflicts);
+
+                var mergedGate = strategy == PolicyMergeStrategy.BaseWins
+                    ? gate with { Config = mergedConfig }
+                    : overlayGate with { Config = mergedConfig };
+
+                result.Add(mergedGate);
+            }
+            else
+            {
+                result.Add(gate);
+            }
+        }
+
+        // Add overlay-only gates
+        foreach (var gate in overlayGates.Where(g => !baseMap.ContainsKey(g.Id)))
+        {
+            result.Add(gate);
+        }
+
+        return result;
+    }
+
+    private static List<PolicyRuleDefinition> MergeRules(
+        IReadOnlyList<PolicyRuleDefinition> baseRules,
+        IReadOnlyList<PolicyRuleDefinition> overlayRules,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        var baseMap = baseRules.ToDictionary(r => r.Name);
+        var overlayMap = overlayRules.ToDictionary(r => r.Name);
+        var result = new List<PolicyRuleDefinition>();
+
+        // Include all base rules (potentially overridden)
+        foreach (var rule in baseRules)
+        {
+            if (overlayMap.TryGetValue(rule.Name, out var overlayRule))
+            {
+                // Rule exists in both: pick winner
+                if (strategy == PolicyMergeStrategy.FailOnConflict &&
+                    (rule.Action != overlayRule.Action || rule.Priority != overlayRule.Priority))
+                {
+                    conflicts.Add(new PolicyMergeConflict
+                    {
+                        Path = $"spec.rules[{rule.Name}]",
+                        BaseValue = $"action={rule.Action}, priority={rule.Priority}",
+                        OverlayValue = $"action={overlayRule.Action}, priority={overlayRule.Priority}",
+                        Description = $"Rule '{rule.Name}' differs between base and overlay."
+                    });
+                    result.Add(rule); // Keep base on conflict
+                }
+                else
+                {
+                    result.Add(strategy == PolicyMergeStrategy.BaseWins ? rule : overlayRule);
+                }
+            }
+            else
+            {
+                result.Add(rule);
+            }
+        }
+
+        // Add overlay-only rules
+        foreach (var rule in overlayRules.Where(r => !baseMap.ContainsKey(r.Name)))
+        {
+            result.Add(rule);
+        }
+
+        return result;
+    }
+
+    private static Dictionary<string, object?> MergeDictionaries(
+        IReadOnlyDictionary<string, object?> baseDict,
+        IReadOnlyDictionary<string, object?> overlayDict,
+        string pathPrefix,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        var result = new Dictionary<string, object?>(baseDict);
+
+        foreach (var (key, overlayVal) in overlayDict)
+        {
+            if (result.TryGetValue(key, out var baseVal) && !ValuesEqual(baseVal, overlayVal))
+            {
+                if (strategy == PolicyMergeStrategy.FailOnConflict)
+                {
+                    conflicts.Add(new PolicyMergeConflict
+                    {
+                        Path = $"{pathPrefix}.{key}",
+                        BaseValue = baseVal,
+                        OverlayValue = overlayVal,
+                        Description = $"Config key '{key}' differs: base='{baseVal}', overlay='{overlayVal}'."
+                    });
+                }
+                else if (strategy == PolicyMergeStrategy.OverlayWins)
+                {
+                    result[key] = overlayVal;
+                }
+                // BaseWins: keep existing value
+            }
+            else if (!result.ContainsKey(key))
+            {
+                result[key] = overlayVal;
+            }
+        }
+
+        return result;
+    }
+
+    private static string? ResolveConflict(
+        string baseVal,
+        string overlayVal,
+        string path,
+        PolicyMergeStrategy strategy,
+        List<PolicyMergeConflict> conflicts)
+    {
+        if (baseVal == overlayVal) return baseVal;
+
+        if (strategy == PolicyMergeStrategy.FailOnConflict)
+        {
+            conflicts.Add(new PolicyMergeConflict
+            {
+                Path = path,
+                BaseValue = baseVal,
+                OverlayValue = overlayVal,
+                Description = $"Conflict at '{path}': base='{baseVal}', overlay='{overlayVal}'."
+            });
+            return baseVal;
+        }
+
+        return strategy == PolicyMergeStrategy.OverlayWins ? overlayVal : baseVal;
+    }
+
+    #endregion
+}

src/Policy/__Libraries/StellaOps.Policy.Interop/Export/YamlPolicyExporter.cs

@@ -0,0 +1,265 @@
+// -----------------------------------------------------------------------------
+// YamlPolicyExporter.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - YAML export support for PolicyPackDocument
+// Description: Exports PolicyPackDocuments to canonical YAML format.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Interop.Abstractions;
+using StellaOps.Policy.Interop.Contracts;
+using System.Security.Cryptography;
+using System.Text;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NamingConventions;
+
+namespace StellaOps.Policy.Interop.Export;
+
+/// <summary>
+/// Exports PolicyPackDocuments to canonical YAML format.
+/// Output is deterministic: same input produces byte-identical output
+/// (camelCase keys, sorted properties, consistent formatting).
+/// </summary>
+public sealed class YamlPolicyExporter : IPolicyYamlExporter
+{
+    private static readonly ISerializer YamlSerializer = new SerializerBuilder()
+        .WithNamingConvention(CamelCaseNamingConvention.Instance)
+        .DisableAliases()
+        .ConfigureDefaultValuesHandling(DefaultValuesHandling.OmitNull)
+        .Build();
+
+    private static readonly IDeserializer YamlDeserializer = new DeserializerBuilder()
+        .WithNamingConvention(CamelCaseNamingConvention.Instance)
+        .Build();
+
+    /// <inheritdoc/>
+    public Task<YamlExportResult> ExportToYamlAsync(
+        PolicyPackDocument document,
+        PolicyExportRequest request,
+        CancellationToken ct = default)
+    {
+        var exported = document;
+
+        // Apply environment filter if specified
+        if (request.Environment is not null)
+        {
+            exported = FilterByEnvironment(exported, request.Environment);
+        }
+
+        // Strip remediation if not requested
+        if (!request.IncludeRemediation)
+        {
+            exported = StripRemediation(exported);
+        }
+
+        // Serialize to YAML
+        var yamlContent = SerializeToYaml(exported);
+
+        // Compute digest
+        var digest = ComputeDigest(yamlContent);
+
+        return Task.FromResult(new YamlExportResult
+        {
+            Success = true,
+            YamlContent = yamlContent,
+            Digest = digest,
+            Warnings = []
+        });
+    }
+
+    /// <summary>
+    /// Serializes a PolicyPackDocument to canonical YAML string.
+    /// </summary>
+    public static string SerializeToYaml(PolicyPackDocument document)
+    {
+        // Convert to an intermediate dictionary to ensure consistent output
+        var intermediate = ConvertToSerializable(document);
+        return YamlSerializer.Serialize(intermediate);
+    }
+
+    /// <summary>
+    /// Serializes a PolicyPackDocument to canonical YAML bytes (UTF-8).
+    /// </summary>
+    public static byte[] SerializeCanonical(PolicyPackDocument document)
+    {
+        return Encoding.UTF8.GetBytes(SerializeToYaml(document));
+    }
+
+    private static object ConvertToSerializable(PolicyPackDocument doc)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["apiVersion"] = doc.ApiVersion,
+            ["kind"] = doc.Kind,
+            ["metadata"] = ConvertMetadata(doc.Metadata),
+            ["spec"] = ConvertSpec(doc.Spec)
+        };
+        return result;
+    }
+
+    private static object ConvertMetadata(PolicyPackMetadata meta)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["name"] = meta.Name,
+            ["version"] = meta.Version
+        };
+
+        if (meta.Description is not null) result["description"] = meta.Description;
+        if (meta.Digest is not null) result["digest"] = meta.Digest;
+        if (meta.CreatedAt is not null) result["createdAt"] = meta.CreatedAt.Value.ToString("O");
+        if (meta.Parent is not null) result["parent"] = meta.Parent;
+        if (meta.Environment is not null) result["environment"] = meta.Environment;
+
+        if (meta.ExportedFrom is not null)
+        {
+            var provenance = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+            {
+                ["engine"] = meta.ExportedFrom.Engine,
+                ["engineVersion"] = meta.ExportedFrom.EngineVersion
+            };
+            if (meta.ExportedFrom.ExportedAt is not null)
+                provenance["exportedAt"] = meta.ExportedFrom.ExportedAt.Value.ToString("O");
+            result["exportedFrom"] = provenance;
+        }
+
+        return result;
+    }
+
+    private static object ConvertSpec(PolicyPackSpec spec)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["settings"] = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+            {
+                ["defaultAction"] = spec.Settings.DefaultAction,
+                ["deterministicMode"] = spec.Settings.DeterministicMode,
+                ["stopOnFirstFailure"] = spec.Settings.StopOnFirstFailure,
+                ["unknownsThreshold"] = spec.Settings.UnknownsThreshold
+            },
+            ["gates"] = spec.Gates.Select(ConvertGate).ToList(),
+            ["rules"] = spec.Rules.Select(ConvertRule).ToList()
+        };
+        return result;
+    }
+
+    private static object ConvertGate(PolicyGateDefinition gate)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["id"] = gate.Id,
+            ["type"] = gate.Type,
+            ["enabled"] = gate.Enabled
+        };
+
+        if (gate.Config.Count > 0)
+            result["config"] = new SortedDictionary<string, object?>(gate.Config.ToDictionary(kv => kv.Key, kv => kv.Value), StringComparer.Ordinal);
+
+        if (gate.Environments is not null)
+        {
+            var envs = new SortedDictionary<string, object?>(StringComparer.Ordinal);
+            foreach (var (env, cfg) in gate.Environments.OrderBy(e => e.Key, StringComparer.Ordinal))
+            {
+                envs[env] = new SortedDictionary<string, object?>(cfg.ToDictionary(kv => kv.Key, kv => kv.Value), StringComparer.Ordinal);
+            }
+            result["environments"] = envs;
+        }
+
+        if (gate.Remediation is not null)
+            result["remediation"] = ConvertRemediation(gate.Remediation);
+
+        return result;
+    }
+
+    private static object ConvertRule(PolicyRuleDefinition rule)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["name"] = rule.Name,
+            ["action"] = rule.Action,
+            ["priority"] = rule.Priority
+        };
+
+        if (rule.Match.Count > 0)
+            result["match"] = new SortedDictionary<string, object?>(rule.Match.ToDictionary(kv => kv.Key, kv => kv.Value), StringComparer.Ordinal);
+
+        if (rule.Remediation is not null)
+            result["remediation"] = ConvertRemediation(rule.Remediation);
+
+        return result;
+    }
+
+    private static object ConvertRemediation(RemediationHint hint)
+    {
+        var result = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+        {
+            ["code"] = hint.Code,
+            ["title"] = hint.Title
+        };
+        if (hint.Description is not null) result["description"] = hint.Description;
+        if (hint.Actions.Count > 0)
+        {
+            result["actions"] = hint.Actions.Select(a =>
+            {
+                var actionDict = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+                {
+                    ["type"] = a.Type
+                };
+                if (a.Description is not null) actionDict["description"] = a.Description;
+                if (a.Command is not null) actionDict["command"] = a.Command;
+                return (object)actionDict;
+            }).ToList();
+        }
+        if (hint.References.Count > 0)
+        {
+            result["references"] = hint.References.Select(r =>
+            {
+                var refDict = new SortedDictionary<string, object?>(StringComparer.Ordinal)
+                {
+                    ["title"] = r.Title
+                };
+                if (r.Url is not null) refDict["url"] = r.Url;
+                return (object)refDict;
+            }).ToList();
+        }
+        return result;
+    }
+
+    private static PolicyPackDocument FilterByEnvironment(PolicyPackDocument doc, string environment)
+    {
+        var filteredGates = doc.Spec.Gates.Select(g =>
+        {
+            if (g.Environments is null || !g.Environments.ContainsKey(environment))
+                return g;
+
+            var envConfig = g.Environments[environment];
+            var mergedConfig = new Dictionary<string, object?>(g.Config);
+            foreach (var (key, value) in envConfig)
+            {
+                mergedConfig[key] = value;
+            }
+
+            return g with { Config = mergedConfig, Environments = null };
+        }).ToList();
+
+        return doc with
+        {
+            Spec = doc.Spec with { Gates = filteredGates }
+        };
+    }
+
+    private static PolicyPackDocument StripRemediation(PolicyPackDocument doc)
+    {
+        var gates = doc.Spec.Gates.Select(g => g with { Remediation = null }).ToList();
+        var rules = doc.Spec.Rules.Select(r => r with { Remediation = null }).ToList();
+        return doc with
+        {
+            Spec = doc.Spec with { Gates = gates, Rules = rules }
+        };
+    }
+
+    private static string ComputeDigest(string content)
+    {
+        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(content));
+        return $"sha256:{Convert.ToHexStringLower(hash)}";
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Interop/Import/FormatDetector.cs

@@ -45,6 +45,15 @@ public static class FormatDetector
             return PolicyFormats.Rego;
         }
 
+        // YAML detection: starts with apiVersion: or --- or has YAML-like key: value structure
+        if (trimmed.StartsWith("---", StringComparison.Ordinal) ||
+            trimmed.StartsWith("apiVersion:", StringComparison.Ordinal) ||
+            (trimmed.Contains("apiVersion:", StringComparison.Ordinal) &&
+             trimmed.Contains("kind:", StringComparison.Ordinal)))
+        {
+            return PolicyFormats.Yaml;
+        }
+
         return null;
     }
 
@@ -58,6 +67,7 @@ public static class FormatDetector
         {
             ".json" => PolicyFormats.Json,
             ".rego" => PolicyFormats.Rego,
+            ".yaml" or ".yml" => PolicyFormats.Yaml,
             _ => null
         };
     }

src/Policy/__Libraries/StellaOps.Policy.Interop/Import/YamlPolicyImporter.cs

@@ -0,0 +1,137 @@
+// -----------------------------------------------------------------------------
+// YamlPolicyImporter.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - YAML import support for PolicyPackDocument
+// Description: Imports PolicyPackDocuments from YAML format.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Interop.Abstractions;
+using StellaOps.Policy.Interop.Contracts;
+using System.Text;
+using System.Text.Json;
+using YamlDotNet.Core;
+using YamlDotNet.Serialization;
+using YamlDotNet.Serialization.NamingConventions;
+
+namespace StellaOps.Policy.Interop.Import;
+
+/// <summary>
+/// Imports PolicyPack v2 YAML documents into the native model.
+/// Converts YAML to JSON intermediary then delegates to the JSON importer for validation.
+/// This ensures consistent validation behavior across all import formats.
+/// </summary>
+public sealed class YamlPolicyImporter : IPolicyImporter
+{
+    private static readonly IDeserializer YamlDeserializer = new DeserializerBuilder()
+        .WithNamingConvention(CamelCaseNamingConvention.Instance)
+        .Build();
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        PropertyNameCaseInsensitive = true,
+        AllowTrailingCommas = true,
+        ReadCommentHandling = JsonCommentHandling.Skip
+    };
+
+    private readonly JsonPolicyImporter _jsonImporter = new();
+
+    /// <inheritdoc/>
+    public async Task<PolicyImportResult> ImportAsync(
+        Stream policyStream,
+        PolicyImportOptions options,
+        CancellationToken ct = default)
+    {
+        using var reader = new StreamReader(policyStream, Encoding.UTF8);
+        var content = await reader.ReadToEndAsync(ct);
+        return await ImportFromStringAsync(content, options, ct);
+    }
+
+    /// <inheritdoc/>
+    public Task<PolicyImportResult> ImportFromStringAsync(
+        string content,
+        PolicyImportOptions options,
+        CancellationToken ct = default)
+    {
+        // Parse YAML into object graph
+        object? yamlObject;
+        try
+        {
+            yamlObject = YamlDeserializer.Deserialize<object>(content);
+        }
+        catch (YamlException ex)
+        {
+            return Task.FromResult(new PolicyImportResult
+            {
+                Success = false,
+                DetectedFormat = PolicyFormats.Yaml,
+                Diagnostics =
+                [
+                    new PolicyDiagnostic
+                    {
+                        Severity = PolicyDiagnostic.Severities.Error,
+                        Code = "YAML_PARSE_ERROR",
+                        Message = $"YAML parse error at line {ex.Start.Line}, column {ex.Start.Column}: {ex.Message}",
+                        Location = $"line {ex.Start.Line}, column {ex.Start.Column}"
+                    }
+                ]
+            });
+        }
+
+        if (yamlObject is null)
+        {
+            return Task.FromResult(new PolicyImportResult
+            {
+                Success = false,
+                DetectedFormat = PolicyFormats.Yaml,
+                Diagnostics =
+                [
+                    new PolicyDiagnostic
+                    {
+                        Severity = PolicyDiagnostic.Severities.Error,
+                        Code = "YAML_EMPTY",
+                        Message = "YAML document is empty or null."
+                    }
+                ]
+            });
+        }
+
+        // Convert YAML object graph to JSON string (YamlDotNet -> System.Text.Json roundtrip)
+        string jsonContent;
+        try
+        {
+            jsonContent = JsonSerializer.Serialize(yamlObject, JsonOptions);
+        }
+        catch (Exception ex)
+        {
+            return Task.FromResult(new PolicyImportResult
+            {
+                Success = false,
+                DetectedFormat = PolicyFormats.Yaml,
+                Diagnostics =
+                [
+                    new PolicyDiagnostic
+                    {
+                        Severity = PolicyDiagnostic.Severities.Error,
+                        Code = "YAML_CONVERSION_ERROR",
+                        Message = $"Failed to convert YAML to JSON: {ex.Message}"
+                    }
+                ]
+            });
+        }
+
+        // Delegate to JSON importer for validation and deserialization
+        var jsonOptions = options with { Format = PolicyFormats.Json };
+        var result = _jsonImporter.ImportFromStringAsync(jsonContent, jsonOptions, ct);
+
+        return result.ContinueWith(t =>
+        {
+            var importResult = t.Result;
+
+            // Update detected format to YAML
+            return importResult with
+            {
+                DetectedFormat = PolicyFormats.Yaml
+            };
+        }, ct, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
+    }
+}

src/Policy/__Libraries/StellaOps.Policy.Interop/StellaOps.Policy.Interop.csproj

@@ -13,6 +13,7 @@
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
     <PackageReference Include="Microsoft.Extensions.Options" />
     <PackageReference Include="JsonSchema.Net" />
+    <PackageReference Include="YamlDotNet" />
   </ItemGroup>
 
   <ItemGroup>

src/Policy/__Libraries/__Tests/StellaOps.Policy.Interop.Tests/DiffMerge/PolicyDiffMergeEngineTests.cs

@@ -0,0 +1,421 @@
+// -----------------------------------------------------------------------------
+// PolicyDiffMergeEngineTests.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - Tests for diff/merge engine
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using System.Text.Json;
+using FluentAssertions;
+using StellaOps.Policy.Interop.Abstractions;
+using StellaOps.Policy.Interop.Contracts;
+using StellaOps.Policy.Interop.DiffMerge;
+using Xunit;
+
+namespace StellaOps.Policy.Interop.Tests.DiffMerge;
+
+public sealed class PolicyDiffMergeEngineTests
+{
+    private readonly PolicyDiffMergeEngine _engine = new();
+
+    private static PolicyPackDocument LoadGoldenFixture()
+    {
+        var fixturePath = Path.Combine(AppContext.BaseDirectory, "Fixtures", "golden-policy-pack-v2.json");
+        var json = File.ReadAllText(fixturePath);
+        return JsonSerializer.Deserialize<PolicyPackDocument>(json,
+            new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;
+    }
+
+    private static PolicyPackDocument CreateMinimalDoc(
+        string name = "test", string version = "1.0.0", string defaultAction = "block")
+    {
+        return new PolicyPackDocument
+        {
+            ApiVersion = PolicyPackDocument.ApiVersionV2,
+            Kind = PolicyPackDocument.KindPolicyPack,
+            Metadata = new PolicyPackMetadata { Name = name, Version = version },
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = defaultAction },
+                Gates = [],
+                Rules = []
+            }
+        };
+    }
+
+    #region Diff Tests
+
+    [Fact]
+    public void Diff_IdenticalDocuments_ReturnsNoChanges()
+    {
+        var doc = LoadGoldenFixture();
+
+        var result = _engine.Diff(doc, doc);
+
+        result.AreIdentical.Should().BeTrue();
+        result.Changes.Should().BeEmpty();
+        result.Summary.Total.Should().Be(0);
+    }
+
+    [Fact]
+    public void Diff_MetadataVersionChange_DetectsModification()
+    {
+        var baseline = CreateMinimalDoc(version: "1.0.0");
+        var updated = baseline with
+        {
+            Metadata = baseline.Metadata with { Version = "2.0.0" }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.AreIdentical.Should().BeFalse();
+        result.Summary.Modifications.Should().Be(1);
+        result.Changes.Should().ContainSingle(c =>
+            c.Path == "metadata.version" && c.ChangeType == PolicyChangeType.Modified);
+    }
+
+    [Fact]
+    public void Diff_SettingsChange_DetectsDefaultActionModification()
+    {
+        var baseline = CreateMinimalDoc(defaultAction: "block");
+        var updated = baseline with
+        {
+            Spec = baseline.Spec with
+            {
+                Settings = baseline.Spec.Settings with { DefaultAction = "warn" }
+            }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.AreIdentical.Should().BeFalse();
+        result.Changes.Should().ContainSingle(c =>
+            c.Path == "spec.settings.defaultAction" &&
+            c.OldValue!.ToString() == "block" &&
+            c.NewValue!.ToString() == "warn");
+    }
+
+    [Fact]
+    public void Diff_GateAdded_DetectsAddition()
+    {
+        var baseline = CreateMinimalDoc();
+        var updated = baseline with
+        {
+            Spec = baseline.Spec with
+            {
+                Gates =
+                [
+                    new PolicyGateDefinition
+                    {
+                        Id = "new-gate",
+                        Type = "CvssThresholdGate"
+                    }
+                ]
+            }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.Summary.Additions.Should().Be(1);
+        result.Changes.Should().ContainSingle(c =>
+            c.ChangeType == PolicyChangeType.Added && c.Category == "gate");
+    }
+
+    [Fact]
+    public void Diff_GateRemoved_DetectsRemoval()
+    {
+        var baseline = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates =
+                [
+                    new PolicyGateDefinition
+                    {
+                        Id = "old-gate",
+                        Type = "SbomPresenceGate"
+                    }
+                ],
+                Rules = []
+            }
+        };
+        var updated = baseline with
+        {
+            Spec = baseline.Spec with { Gates = [] }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.Summary.Removals.Should().Be(1);
+        result.Changes.Should().ContainSingle(c =>
+            c.ChangeType == PolicyChangeType.Removed && c.Category == "gate");
+    }
+
+    [Fact]
+    public void Diff_RuleActionChanged_DetectsModification()
+    {
+        var rule = new PolicyRuleDefinition
+        {
+            Name = "test-rule",
+            Action = "block",
+            Priority = 10
+        };
+
+        var baseline = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates = [],
+                Rules = [rule]
+            }
+        };
+        var updated = baseline with
+        {
+            Spec = baseline.Spec with
+            {
+                Rules = [rule with { Action = "warn" }]
+            }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.Changes.Should().Contain(c =>
+            c.Path == "spec.rules[test-rule].action" && c.ChangeType == PolicyChangeType.Modified);
+    }
+
+    [Fact]
+    public void Diff_GateConfigChanged_DetectsConfigModification()
+    {
+        var gate = new PolicyGateDefinition
+        {
+            Id = "cvss-gate",
+            Type = "CvssThresholdGate",
+            Config = new Dictionary<string, object?> { ["threshold"] = (JsonElement)JsonDocument.Parse("7.0").RootElement.Clone() }
+        };
+
+        var baseline = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates = [gate],
+                Rules = []
+            }
+        };
+
+        var updatedGate = gate with
+        {
+            Config = new Dictionary<string, object?> { ["threshold"] = (JsonElement)JsonDocument.Parse("9.0").RootElement.Clone() }
+        };
+        var updated = baseline with
+        {
+            Spec = baseline.Spec with { Gates = [updatedGate] }
+        };
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.Changes.Should().Contain(c =>
+            c.Path == "spec.gates[cvss-gate].config.threshold" && c.ChangeType == PolicyChangeType.Modified);
+    }
+
+    [Fact]
+    public void Diff_GoldenFixture_AgainstItself_IsIdentical()
+    {
+        var doc = LoadGoldenFixture();
+
+        var result = _engine.Diff(doc, doc);
+
+        result.AreIdentical.Should().BeTrue();
+    }
+
+    [Fact]
+    public void Diff_MultipleChanges_ReturnsCorrectSummary()
+    {
+        var baseline = CreateMinimalDoc(name: "base", version: "1.0.0", defaultAction: "block");
+        var updated = CreateMinimalDoc(name: "updated", version: "2.0.0", defaultAction: "warn");
+
+        var result = _engine.Diff(baseline, updated);
+
+        result.Summary.Modifications.Should().Be(3); // name, version, defaultAction
+        result.Summary.Total.Should().Be(3);
+    }
+
+    #endregion
+
+    #region Merge Tests
+
+    [Fact]
+    public void Merge_IdenticalDocuments_ReturnsIdenticalResult()
+    {
+        var doc = CreateMinimalDoc();
+
+        var result = _engine.Merge(doc, doc);
+
+        result.Success.Should().BeTrue();
+        result.Document.Should().NotBeNull();
+        result.Conflicts.Should().BeEmpty();
+    }
+
+    [Fact]
+    public void Merge_OverlayWins_OverlayValuesPreferred()
+    {
+        var baseDoc = CreateMinimalDoc(defaultAction: "block");
+        var overlay = CreateMinimalDoc(defaultAction: "warn");
+
+        var result = _engine.Merge(baseDoc, overlay, PolicyMergeStrategy.OverlayWins);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Settings.DefaultAction.Should().Be("warn");
+    }
+
+    [Fact]
+    public void Merge_BaseWins_BaseValuesPreferred()
+    {
+        var baseDoc = CreateMinimalDoc(defaultAction: "block");
+        var overlay = CreateMinimalDoc(defaultAction: "warn");
+
+        var result = _engine.Merge(baseDoc, overlay, PolicyMergeStrategy.BaseWins);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Settings.DefaultAction.Should().Be("block");
+    }
+
+    [Fact]
+    public void Merge_FailOnConflict_ReportsConflicts()
+    {
+        var baseDoc = CreateMinimalDoc(defaultAction: "block");
+        var overlay = CreateMinimalDoc(defaultAction: "warn");
+
+        var result = _engine.Merge(baseDoc, overlay, PolicyMergeStrategy.FailOnConflict);
+
+        result.Success.Should().BeFalse();
+        result.Conflicts.Should().NotBeEmpty();
+    }
+
+    [Fact]
+    public void Merge_OverlayAddsNewGate_GateIncluded()
+    {
+        var baseDoc = CreateMinimalDoc();
+        var overlay = baseDoc with
+        {
+            Spec = baseDoc.Spec with
+            {
+                Gates =
+                [
+                    new PolicyGateDefinition
+                    {
+                        Id = "overlay-gate",
+                        Type = "CvssThresholdGate"
+                    }
+                ]
+            }
+        };
+
+        var result = _engine.Merge(baseDoc, overlay);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Gates.Should().ContainSingle(g => g.Id == "overlay-gate");
+    }
+
+    [Fact]
+    public void Merge_OverlayAddsNewRule_RuleIncluded()
+    {
+        var baseDoc = CreateMinimalDoc();
+        var overlay = baseDoc with
+        {
+            Spec = baseDoc.Spec with
+            {
+                Rules =
+                [
+                    new PolicyRuleDefinition
+                    {
+                        Name = "overlay-rule",
+                        Action = "warn",
+                        Priority = 50
+                    }
+                ]
+            }
+        };
+
+        var result = _engine.Merge(baseDoc, overlay);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Rules.Should().ContainSingle(r => r.Name == "overlay-rule");
+    }
+
+    [Fact]
+    public void Merge_BothHaveGates_MergesAllGates()
+    {
+        var baseDoc = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates =
+                [
+                    new PolicyGateDefinition { Id = "base-gate", Type = "SbomPresenceGate" }
+                ],
+                Rules = []
+            }
+        };
+        var overlay = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates =
+                [
+                    new PolicyGateDefinition { Id = "overlay-gate", Type = "CvssThresholdGate" }
+                ],
+                Rules = []
+            }
+        };
+
+        var result = _engine.Merge(baseDoc, overlay);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Gates.Should().HaveCount(2);
+        result.Document.Spec.Gates.Should().Contain(g => g.Id == "base-gate");
+        result.Document.Spec.Gates.Should().Contain(g => g.Id == "overlay-gate");
+    }
+
+    [Fact]
+    public void Merge_OverlayWins_OverridesMatchingGate()
+    {
+        var gate = new PolicyGateDefinition
+        {
+            Id = "shared-gate",
+            Type = "CvssThresholdGate",
+            Enabled = true
+        };
+
+        var baseDoc = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates = [gate],
+                Rules = []
+            }
+        };
+        var overlay = CreateMinimalDoc() with
+        {
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "block" },
+                Gates = [gate with { Enabled = false }],
+                Rules = []
+            }
+        };
+
+        var result = _engine.Merge(baseDoc, overlay, PolicyMergeStrategy.OverlayWins);
+
+        result.Success.Should().BeTrue();
+        result.Document!.Spec.Gates.Should().ContainSingle(g =>
+            g.Id == "shared-gate" && !g.Enabled);
+    }
+
+    #endregion
+}

src/Policy/__Libraries/__Tests/StellaOps.Policy.Interop.Tests/Export/YamlPolicyExporterTests.cs

@@ -0,0 +1,151 @@
+// -----------------------------------------------------------------------------
+// YamlPolicyExporterTests.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - Tests for YAML export
+// -----------------------------------------------------------------------------
+
+using System.Text.Json;
+using FluentAssertions;
+using StellaOps.Policy.Interop.Contracts;
+using StellaOps.Policy.Interop.Export;
+using Xunit;
+
+namespace StellaOps.Policy.Interop.Tests.Export;
+
+public sealed class YamlPolicyExporterTests
+{
+    private readonly YamlPolicyExporter _exporter = new();
+
+    private static PolicyPackDocument LoadGoldenFixture()
+    {
+        var fixturePath = Path.Combine(AppContext.BaseDirectory, "Fixtures", "golden-policy-pack-v2.json");
+        var json = File.ReadAllText(fixturePath);
+        return JsonSerializer.Deserialize<PolicyPackDocument>(json,
+            new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;
+    }
+
+    [Fact]
+    public async Task ExportToYaml_ProducesValidOutput()
+    {
+        var doc = LoadGoldenFixture();
+        var request = new PolicyExportRequest { Format = PolicyFormats.Yaml };
+
+        var result = await _exporter.ExportToYamlAsync(doc, request);
+
+        result.Success.Should().BeTrue();
+        result.YamlContent.Should().NotBeNullOrEmpty();
+        result.Digest.Should().StartWith("sha256:");
+    }
+
+    [Fact]
+    public async Task ExportToYaml_ContainsApiVersionAndKind()
+    {
+        var doc = LoadGoldenFixture();
+        var request = new PolicyExportRequest { Format = PolicyFormats.Yaml };
+
+        var result = await _exporter.ExportToYamlAsync(doc, request);
+
+        result.YamlContent.Should().Contain("apiVersion: policy.stellaops.io/v2");
+        result.YamlContent.Should().Contain("kind: PolicyPack");
+    }
+
+    [Fact]
+    public async Task ExportToYaml_IsDeterministic()
+    {
+        var doc = LoadGoldenFixture();
+        var request = new PolicyExportRequest { Format = PolicyFormats.Yaml };
+
+        var result1 = await _exporter.ExportToYamlAsync(doc, request);
+        var result2 = await _exporter.ExportToYamlAsync(doc, request);
+
+        result1.Digest.Should().Be(result2.Digest);
+        result1.YamlContent.Should().Be(result2.YamlContent);
+    }
+
+    [Fact]
+    public async Task ExportToYaml_WithEnvironment_MergesConfig()
+    {
+        var doc = LoadGoldenFixture();
+        var request = new PolicyExportRequest { Format = PolicyFormats.Yaml, Environment = "staging" };
+
+        var result = await _exporter.ExportToYamlAsync(doc, request);
+
+        // Environment-specific config is merged; environments key should not appear
+        result.YamlContent.Should().NotContain("environments:");
+    }
+
+    [Fact]
+    public async Task ExportToYaml_WithoutRemediation_StripsHints()
+    {
+        var doc = LoadGoldenFixture();
+        var request = new PolicyExportRequest { Format = PolicyFormats.Yaml, IncludeRemediation = false };
+
+        var result = await _exporter.ExportToYamlAsync(doc, request);
+
+        result.YamlContent.Should().NotContain("remediation:");
+    }
+
+    [Fact]
+    public void SerializeCanonical_ProducesDeterministicBytes()
+    {
+        var doc = LoadGoldenFixture();
+
+        var bytes1 = YamlPolicyExporter.SerializeCanonical(doc);
+        var bytes2 = YamlPolicyExporter.SerializeCanonical(doc);
+
+        bytes1.Should().BeEquivalentTo(bytes2);
+    }
+
+    [Fact]
+    public void SerializeToYaml_PreservesGateIds()
+    {
+        var doc = LoadGoldenFixture();
+
+        var yaml = YamlPolicyExporter.SerializeToYaml(doc);
+
+        yaml.Should().Contain("cvss-threshold");
+        yaml.Should().Contain("signature-required");
+        yaml.Should().Contain("evidence-freshness");
+        yaml.Should().Contain("sbom-presence");
+        yaml.Should().Contain("minimum-confidence");
+    }
+
+    [Fact]
+    public void SerializeToYaml_PreservesRuleNames()
+    {
+        var doc = LoadGoldenFixture();
+
+        var yaml = YamlPolicyExporter.SerializeToYaml(doc);
+
+        yaml.Should().Contain("require-dsse-signature");
+        yaml.Should().Contain("require-rekor-proof");
+        yaml.Should().Contain("require-sbom-digest");
+        yaml.Should().Contain("require-freshness-tst");
+    }
+
+    [Fact]
+    public void SerializeToYaml_MinimalDocument_Succeeds()
+    {
+        var doc = new PolicyPackDocument
+        {
+            ApiVersion = PolicyPackDocument.ApiVersionV2,
+            Kind = PolicyPackDocument.KindPolicyPack,
+            Metadata = new PolicyPackMetadata
+            {
+                Name = "minimal",
+                Version = "1.0.0"
+            },
+            Spec = new PolicyPackSpec
+            {
+                Settings = new PolicyPackSettings { DefaultAction = "allow" },
+                Gates = [],
+                Rules = []
+            }
+        };
+
+        var yaml = YamlPolicyExporter.SerializeToYaml(doc);
+
+        yaml.Should().Contain("name: minimal");
+        yaml.Should().Contain("defaultAction: allow");
+    }
+}

src/Policy/__Libraries/__Tests/StellaOps.Policy.Interop.Tests/Import/FormatDetectorTests.cs

@@ -79,8 +79,30 @@ public class FormatDetectorTests
     [Fact]
     public void DetectFromExtension_UnknownExtension_ReturnsNull()
     {
-        FormatDetector.DetectFromExtension("policy.yaml").Should().BeNull();
         FormatDetector.DetectFromExtension("policy.txt").Should().BeNull();
+        FormatDetector.DetectFromExtension("policy.xml").Should().BeNull();
+    }
+
+    [Fact]
+    public void DetectFromExtension_YamlFile_ReturnsYaml()
+    {
+        FormatDetector.DetectFromExtension("policy.yaml").Should().Be(PolicyFormats.Yaml);
+        FormatDetector.DetectFromExtension("policy.yml").Should().Be(PolicyFormats.Yaml);
+        FormatDetector.DetectFromExtension("/path/to/my-policy.yaml").Should().Be(PolicyFormats.Yaml);
+    }
+
+    [Fact]
+    public void Detect_YamlContent_WithApiVersion_ReturnsYaml()
+    {
+        var content = "apiVersion: policy.stellaops.io/v2\nkind: PolicyPack\n";
+        FormatDetector.Detect(content).Should().Be(PolicyFormats.Yaml);
+    }
+
+    [Fact]
+    public void Detect_YamlContent_WithDocumentSeparator_ReturnsYaml()
+    {
+        var content = "---\napiVersion: policy.stellaops.io/v2\n";
+        FormatDetector.Detect(content).Should().Be(PolicyFormats.Yaml);
     }
 
     [Fact]

src/Policy/__Libraries/__Tests/StellaOps.Policy.Interop.Tests/Import/YamlPolicyImporterTests.cs

@@ -0,0 +1,150 @@
+// -----------------------------------------------------------------------------
+// YamlPolicyImporterTests.cs
+// Sprint: SPRINT_20260208_048_Policy_policy_interop_framework
+// Task: T1 - Tests for YAML import
+// -----------------------------------------------------------------------------
+
+using System.Text.Json;
+using FluentAssertions;
+using StellaOps.Policy.Interop.Contracts;
+using StellaOps.Policy.Interop.Export;
+using StellaOps.Policy.Interop.Import;
+using Xunit;
+
+namespace StellaOps.Policy.Interop.Tests.Import;
+
+public sealed class YamlPolicyImporterTests
+{
+    private readonly YamlPolicyImporter _importer = new();
+
+    private static PolicyPackDocument LoadGoldenFixture()
+    {
+        var fixturePath = Path.Combine(AppContext.BaseDirectory, "Fixtures", "golden-policy-pack-v2.json");
+        var json = File.ReadAllText(fixturePath);
+        return JsonSerializer.Deserialize<PolicyPackDocument>(json,
+            new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_ValidDocument_Succeeds()
+    {
+        // Export golden fixture to YAML, then re-import
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Success.Should().BeTrue();
+        result.DetectedFormat.Should().Be(PolicyFormats.Yaml);
+        result.Document.Should().NotBeNull();
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_PreservesApiVersion()
+    {
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Document!.ApiVersion.Should().Be(PolicyPackDocument.ApiVersionV2);
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_PreservesGateCount()
+    {
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.GateCount.Should().Be(original.Spec.Gates.Count);
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_PreservesRuleCount()
+    {
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.RuleCount.Should().Be(original.Spec.Rules.Count);
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_InvalidYaml_ReturnsDiagnostic()
+    {
+        var invalidYaml = "invalid: yaml:\n  bad: [\nincomplete";
+
+        var result = await _importer.ImportFromStringAsync(invalidYaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Success.Should().BeFalse();
+        result.DetectedFormat.Should().Be(PolicyFormats.Yaml);
+        result.Diagnostics.Should().Contain(d => d.Code == "YAML_PARSE_ERROR");
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_EmptyContent_ReturnsDiagnostic()
+    {
+        var result = await _importer.ImportFromStringAsync("",
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Success.Should().BeFalse();
+        result.DetectedFormat.Should().Be(PolicyFormats.Yaml);
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_PreservesMetadataName()
+    {
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Document!.Metadata.Name.Should().Be(original.Metadata.Name);
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_MinimalDocument_Succeeds()
+    {
+        var yaml = """
+            apiVersion: policy.stellaops.io/v2
+            kind: PolicyPack
+            metadata:
+              name: test-minimal
+              version: "1.0.0"
+            spec:
+              settings:
+                defaultAction: allow
+              gates: []
+              rules: []
+            """;
+
+        var result = await _importer.ImportFromStringAsync(yaml,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Success.Should().BeTrue();
+        result.Document!.Metadata.Name.Should().Be("test-minimal");
+        result.Document.Spec.Settings.DefaultAction.Should().Be("allow");
+    }
+
+    [Fact]
+    public async Task ImportFromYaml_Stream_Succeeds()
+    {
+        var original = LoadGoldenFixture();
+        var yaml = YamlPolicyExporter.SerializeToYaml(original);
+        using var stream = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(yaml));
+
+        var result = await _importer.ImportAsync(stream,
+            new PolicyImportOptions { Format = PolicyFormats.Yaml });
+
+        result.Success.Should().BeTrue();
+    }
+}

src/Policy/__Libraries/__Tests/StellaOps.Policy.Interop.Tests/StellaOps.Policy.Interop.Tests.csproj

@@ -14,6 +14,7 @@
     <PackageReference Include="FluentAssertions" />
     <PackageReference Include="NSubstitute" />
     <PackageReference Include="JsonSchema.Net" />
+    <PackageReference Include="YamlDotNet" />
   </ItemGroup>
 
   <ItemGroup>

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/CombinedImpactCalculatorTests.cs

@@ -0,0 +1,220 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+public class CombinedImpactCalculatorTests
+{
+    private readonly CombinedImpactCalculator _calculator;
+
+    public CombinedImpactCalculatorTests()
+    {
+        var impactCalculator = new ImpactScoreCalculator(NullLogger<ImpactScoreCalculator>.Instance);
+        var uncertaintyCalculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);
+        _calculator = new CombinedImpactCalculator(
+            impactCalculator,
+            uncertaintyCalculator,
+            NullLogger<CombinedImpactCalculator>.Instance);
+    }
+
+    [Fact]
+    public void Calculate_HighImpactLowUncertainty_ReturnsHighPriority()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Healthcare,
+            FleetPrevalence = 0.9,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 9.8
+        };
+        var signalSnapshot = CreateFullSnapshot();
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot);
+
+        // Assert
+        result.Impact.Score.Should().BeGreaterThan(0.8);
+        result.Uncertainty.Entropy.Should().Be(0.0);
+        result.EffectivePriority.Should().BeGreaterThan(0.8);
+    }
+
+    [Fact]
+    public void Calculate_HighImpactHighUncertainty_ReducesPriority()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Healthcare,
+            FleetPrevalence = 0.9,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 9.8
+        };
+        var signalSnapshot = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", DateTimeOffset.UtcNow);
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot, uncertaintyPenaltyFactor: 0.5);
+
+        // Assert
+        result.Impact.Score.Should().BeGreaterThan(0.8);
+        result.Uncertainty.Entropy.Should().Be(1.0);
+        // Effective = impact * (1 - 1.0 * 0.5) = impact * 0.5
+        result.EffectivePriority.Should().BeLessThan(result.Impact.Score);
+        result.EffectivePriority.Should().BeApproximately(result.Impact.Score * 0.5, 0.01);
+    }
+
+    [Fact]
+    public void Calculate_LowImpactLowUncertainty_ReturnsLowPriority()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Development,
+            DataSensitivity = DataSensitivity.Public,
+            FleetPrevalence = 0.1,
+            SlaTier = SlaTier.NonCritical,
+            CvssScore = 2.0
+        };
+        var signalSnapshot = CreateFullSnapshot();
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot);
+
+        // Assert
+        result.Impact.Score.Should().BeLessThan(0.2);
+        result.Uncertainty.Entropy.Should().Be(0.0);
+        result.EffectivePriority.Should().BeLessThan(0.2);
+    }
+
+    [Fact]
+    public void Calculate_ZeroPenaltyFactor_IgnoresUncertainty()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Healthcare,
+            FleetPrevalence = 0.9,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 9.8
+        };
+        var signalSnapshot = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", DateTimeOffset.UtcNow);
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot, uncertaintyPenaltyFactor: 0.0);
+
+        // Assert
+        result.EffectivePriority.Should().BeApproximately(result.Impact.Score, 0.001);
+    }
+
+    [Fact]
+    public void Calculate_FullPenaltyFactor_MaximumReduction()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Healthcare,
+            FleetPrevalence = 0.9,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 9.8
+        };
+        var signalSnapshot = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", DateTimeOffset.UtcNow);
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot, uncertaintyPenaltyFactor: 1.0);
+
+        // Assert
+        // With 100% entropy and 100% penalty, effective priority = impact * (1 - 1.0) = 0
+        result.EffectivePriority.Should().BeApproximately(0.0, 0.001);
+    }
+
+    [Fact]
+    public void Calculate_IsDeterministic_SameInputSameOutput()
+    {
+        // Arrange
+        var impactContext = new ImpactContext
+        {
+            Environment = EnvironmentType.Staging,
+            DataSensitivity = DataSensitivity.Pii,
+            FleetPrevalence = 0.5,
+            SlaTier = SlaTier.Important,
+            CvssScore = 7.5
+        };
+        var signalSnapshot = CreatePartialSnapshot();
+
+        // Act
+        var result1 = _calculator.Calculate(impactContext, signalSnapshot);
+        var result2 = _calculator.Calculate(impactContext, signalSnapshot);
+
+        // Assert
+        result1.EffectivePriority.Should().Be(result2.EffectivePriority);
+        result1.EffectivePriorityBasisPoints.Should().Be(result2.EffectivePriorityBasisPoints);
+    }
+
+    [Fact]
+    public void Calculate_BasisPointsCalculatedCorrectly()
+    {
+        // Arrange
+        var impactContext = ImpactContext.DefaultForUnknowns();
+        var signalSnapshot = CreateFullSnapshot();
+
+        // Act
+        var result = _calculator.Calculate(impactContext, signalSnapshot);
+
+        // Assert
+        var expectedBasisPoints = (int)Math.Round(result.EffectivePriority * 10000);
+        result.EffectivePriorityBasisPoints.Should().Be(expectedBasisPoints);
+    }
+
+    private SignalSnapshot CreateFullSnapshot()
+    {
+        var now = DateTimeOffset.UtcNow;
+        return new SignalSnapshot
+        {
+            Cve = "CVE-2024-1234",
+            Purl = "pkg:maven/test@1.0",
+            Vex = SignalState<VexClaimSummary>.Queried(
+                new VexClaimSummary { Status = "affected", Confidence = 0.95, StatementCount = 3, ComputedAt = now }, now),
+            Epss = SignalState<EpssEvidence>.Queried(
+                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = now }, now),
+            Reachability = SignalState<ReachabilityEvidence>.Queried(
+                new ReachabilityEvidence { Status = ReachabilityStatus.Reachable, AnalyzedAt = now }, now),
+            Runtime = SignalState<RuntimeEvidence>.Queried(
+                new RuntimeEvidence { Detected = true, DetectedAt = now }, now),
+            Backport = SignalState<BackportEvidence>.Queried(
+                new BackportEvidence { Detected = false, AnalyzedAt = now }, now),
+            Sbom = SignalState<SbomLineageEvidence>.Queried(
+                new SbomLineageEvidence { HasLineage = true, AnalyzedAt = now }, now),
+            Cvss = SignalState<CvssEvidence>.Queried(
+                new CvssEvidence { Version = "3.1", BaseScore = 9.8, Severity = "CRITICAL", Source = "NVD", PublishedAt = now }, now),
+            SnapshotAt = now
+        };
+    }
+
+    private SignalSnapshot CreatePartialSnapshot()
+    {
+        var now = DateTimeOffset.UtcNow;
+        return new SignalSnapshot
+        {
+            Cve = "CVE-2024-1234",
+            Purl = "pkg:maven/test@1.0",
+            Vex = SignalState<VexClaimSummary>.Queried(
+                new VexClaimSummary { Status = "affected", Confidence = 0.95, StatementCount = 3, ComputedAt = now }, now),
+            Epss = SignalState<EpssEvidence>.Queried(
+                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = now }, now),
+            Reachability = SignalState<ReachabilityEvidence>.NotQueried(),
+            Runtime = SignalState<RuntimeEvidence>.NotQueried(),
+            Backport = SignalState<BackportEvidence>.NotQueried(),
+            Sbom = SignalState<SbomLineageEvidence>.NotQueried(),
+            Cvss = SignalState<CvssEvidence>.NotQueried(),
+            SnapshotAt = now
+        };
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/DeltaIfPresentCalculatorTests.cs

@@ -0,0 +1,262 @@
+// <copyright file="DeltaIfPresentCalculatorTests.cs" company="StellaOps">
+// Copyright (c) StellaOps. Licensed under the BUSL-1.1.
+// </copyright>
+
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Time.Testing;
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+public sealed class DeltaIfPresentCalculatorTests
+{
+    private readonly FakeTimeProvider _timeProvider = new();
+    private readonly UncertaintyScoreCalculator _uncertaintyCalculator;
+    private readonly TrustScoreAggregator _trustAggregator;
+    private readonly DeltaIfPresentCalculator _calculator;
+
+    public DeltaIfPresentCalculatorTests()
+    {
+        _uncertaintyCalculator = new UncertaintyScoreCalculator(
+            NullLogger<UncertaintyScoreCalculator>.Instance);
+        _trustAggregator = new TrustScoreAggregator(
+            NullLogger<TrustScoreAggregator>.Instance);
+        _calculator = new DeltaIfPresentCalculator(
+            NullLogger<DeltaIfPresentCalculator>.Instance,
+            _uncertaintyCalculator,
+            _trustAggregator,
+            _timeProvider);
+    }
+
+    [Fact]
+    public void CalculateSingleSignalDelta_VexSignal_ReturnsExpectedDelta()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var result = _calculator.CalculateSingleSignalDelta(snapshot, "VEX", 0.0);
+
+        // Assert
+        result.Signal.Should().Be("VEX");
+        result.AssumedValue.Should().Be(0.0);
+        result.SignalWeight.Should().Be(0.25);
+        result.HypotheticalEntropy.Should().BeLessThan(result.CurrentEntropy);
+    }
+
+    [Fact]
+    public void CalculateSingleSignalDelta_HighRiskValue_IncreasesScore()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var lowRisk = _calculator.CalculateSingleSignalDelta(snapshot, "EPSS", 0.0);
+        var highRisk = _calculator.CalculateSingleSignalDelta(snapshot, "EPSS", 1.0);
+
+        // Assert
+        highRisk.HypotheticalScore.Should().BeGreaterThan(lowRisk.HypotheticalScore);
+    }
+
+    [Fact]
+    public void CalculateSingleSignalDelta_AddsSignal_DecreasesEntropy()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var result = _calculator.CalculateSingleSignalDelta(snapshot, "Runtime", 0.5);
+
+        // Assert
+        result.EntropyDelta.Should().BeLessThan(0);
+        result.HypotheticalEntropy.Should().BeLessThan(result.CurrentEntropy);
+    }
+
+    [Fact]
+    public void CalculateFullAnalysis_ReturnsAllGaps()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var analysis = _calculator.CalculateFullAnalysis(snapshot);
+
+        // Assert
+        analysis.GapAnalysis.Should().HaveCountGreaterThan(0);
+        analysis.PrioritizedGaps.Should().NotBeEmpty();
+        analysis.ComputedAt.Should().Be(_timeProvider.GetUtcNow());
+    }
+
+    [Fact]
+    public void CalculateFullAnalysis_PrioritizesByMaxImpact()
+    {
+        // Arrange
+        var snapshot = CreateEmptySnapshot();
+
+        // Act
+        var analysis = _calculator.CalculateFullAnalysis(snapshot);
+
+        // Assert - VEX and Reachability have highest weights (0.25 each)
+        var topPriority = analysis.PrioritizedGaps.Take(2);
+        topPriority.Should().Contain(s => s == "VEX" || s == "Reachability");
+    }
+
+    [Fact]
+    public void CalculateFullAnalysis_IncludesBestWorstPriorCases()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var analysis = _calculator.CalculateFullAnalysis(snapshot);
+
+        // Assert
+        foreach (var gap in analysis.GapAnalysis)
+        {
+            gap.BestCase.Should().NotBeNull();
+            gap.WorstCase.Should().NotBeNull();
+            gap.PriorCase.Should().NotBeNull();
+
+            gap.BestCase.AssumedValue.Should().Be(0.0);
+            gap.WorstCase.AssumedValue.Should().Be(1.0);
+            gap.MaxImpact.Should().BeGreaterOrEqualTo(0.0);
+        }
+    }
+
+    [Fact]
+    public void CalculateScoreBounds_NoGaps_ReturnsSingleValue()
+    {
+        // Arrange
+        var snapshot = CreateFullSnapshot();
+
+        // Act
+        var bounds = _calculator.CalculateScoreBounds(snapshot);
+
+        // Assert
+        bounds.GapCount.Should().Be(0);
+        bounds.Range.Should().Be(0.0);
+        bounds.MinimumScore.Should().Be(bounds.MaximumScore);
+        bounds.MissingWeightPercentage.Should().Be(0.0);
+    }
+
+    [Fact]
+    public void CalculateScoreBounds_WithGaps_ReturnsRange()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var bounds = _calculator.CalculateScoreBounds(snapshot);
+
+        // Assert
+        bounds.GapCount.Should().BeGreaterThan(0);
+        bounds.Range.Should().BeGreaterThan(0.0);
+        bounds.MaximumScore.Should().BeGreaterThanOrEqualTo(bounds.MinimumScore);
+        bounds.MissingWeightPercentage.Should().BeGreaterThan(0.0);
+    }
+
+    [Fact]
+    public void CalculateScoreBounds_EmptySnapshot_ReturnsFullRange()
+    {
+        // Arrange
+        var snapshot = CreateEmptySnapshot();
+
+        // Act
+        var bounds = _calculator.CalculateScoreBounds(snapshot);
+
+        // Assert
+        bounds.GapCount.Should().Be(6); // All 6 signals missing
+        bounds.CurrentEntropy.Should().Be(1.0);
+        bounds.MissingWeightPercentage.Should().Be(100.0);
+    }
+
+    [Theory]
+    [InlineData("VEX", 0.25)]
+    [InlineData("EPSS", 0.15)]
+    [InlineData("Reachability", 0.25)]
+    [InlineData("Runtime", 0.15)]
+    [InlineData("Backport", 0.10)]
+    [InlineData("SBOMLineage", 0.10)]
+    public void CalculateSingleSignalDelta_CorrectWeightPerSignal(string signal, double expectedWeight)
+    {
+        // Arrange
+        var snapshot = CreateEmptySnapshot();
+
+        // Act
+        var result = _calculator.CalculateSingleSignalDelta(snapshot, signal, 0.5);
+
+        // Assert
+        result.SignalWeight.Should().Be(expectedWeight);
+    }
+
+    [Fact]
+    public void CalculateSingleSignalDelta_DeterministicOutput()
+    {
+        // Arrange
+        var snapshot = CreatePartialSnapshot();
+
+        // Act - Run twice
+        var result1 = _calculator.CalculateSingleSignalDelta(snapshot, "VEX", 0.5);
+        var result2 = _calculator.CalculateSingleSignalDelta(snapshot, "VEX", 0.5);
+
+        // Assert - Results should be identical
+        result1.CurrentScore.Should().Be(result2.CurrentScore);
+        result1.HypotheticalScore.Should().Be(result2.HypotheticalScore);
+        result1.CurrentEntropy.Should().Be(result2.CurrentEntropy);
+        result1.HypotheticalEntropy.Should().Be(result2.HypotheticalEntropy);
+    }
+
+    private SignalSnapshot CreateEmptySnapshot()
+    {
+        return SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow());
+    }
+
+    private SignalSnapshot CreatePartialSnapshot()
+    {
+        var now = _timeProvider.GetUtcNow();
+        return new SignalSnapshot
+        {
+            Cve = "CVE-2024-1234",
+            Purl = "pkg:maven/test@1.0",
+            Vex = SignalState<VexClaimSummary>.NotQueried(),
+            Epss = SignalState<EpssEvidence>.NotQueried(),
+            Reachability = SignalState<ReachabilityEvidence>.Queried(
+                new ReachabilityEvidence { Status = ReachabilityStatus.Reachable, AnalyzedAt = now }, now),
+            Runtime = SignalState<RuntimeEvidence>.NotQueried(),
+            Backport = SignalState<BackportEvidence>.NotQueried(),
+            Sbom = SignalState<SbomLineageEvidence>.Queried(
+                new SbomLineageEvidence { SbomDigest = "sha256:abc", Format = "SPDX", ComponentCount = 150, GeneratedAt = now, HasProvenance = true }, now),
+            Cvss = SignalState<CvssEvidence>.NotQueried(),
+            SnapshotAt = now
+        };
+    }
+
+    private SignalSnapshot CreateFullSnapshot()
+    {
+        var now = _timeProvider.GetUtcNow();
+        return new SignalSnapshot
+        {
+            Cve = "CVE-2024-1234",
+            Purl = "pkg:maven/test@1.0",
+            Vex = SignalState<VexClaimSummary>.Queried(
+                new VexClaimSummary { Status = "affected", Confidence = 0.95, StatementCount = 3, ComputedAt = now }, now),
+            Epss = SignalState<EpssEvidence>.Queried(
+                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = now }, now),
+            Reachability = SignalState<ReachabilityEvidence>.Queried(
+                new ReachabilityEvidence { Status = ReachabilityStatus.Reachable, AnalyzedAt = now }, now),
+            Runtime = SignalState<RuntimeEvidence>.Queried(
+                new RuntimeEvidence { Detected = true, Source = "test", ObservationStart = now.AddDays(-7), ObservationEnd = now, Confidence = 0.9 }, now),
+            Backport = SignalState<BackportEvidence>.Queried(
+                new BackportEvidence { Detected = false, Source = "test", DetectedAt = now, Confidence = 0.85 }, now),
+            Sbom = SignalState<SbomLineageEvidence>.Queried(
+                new SbomLineageEvidence { SbomDigest = "sha256:abc", Format = "SPDX", ComponentCount = 150, GeneratedAt = now, HasProvenance = true }, now),
+            Cvss = SignalState<CvssEvidence>.Queried(
+                new CvssEvidence { Vector = "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H", Version = "3.1", BaseScore = 9.8, Severity = "CRITICAL", Source = "NVD", PublishedAt = now }, now),
+            SnapshotAt = now
+        };
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/EwsCalculatorTests.cs

@@ -0,0 +1,400 @@
+// -----------------------------------------------------------------------------
+// EwsCalculatorTests.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Unit tests for EWS calculator.
+// -----------------------------------------------------------------------------
+
+using Microsoft.Extensions.Time.Testing;
+using StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+public sealed class EwsCalculatorTests
+{
+    private readonly EwsCalculator _calculator;
+    private readonly FakeTimeProvider _timeProvider;
+
+    public EwsCalculatorTests()
+    {
+        _timeProvider = new FakeTimeProvider(DateTimeOffset.Parse("2026-02-08T12:00:00Z"));
+        _calculator = EwsCalculator.CreateDefault(_timeProvider);
+    }
+
+    [Fact]
+    public void Calculate_WithEmptySignal_ReturnsConservativeScore()
+    {
+        // Arrange
+        var signal = EwsSignalInput.Empty;
+
+        // Act
+        var result = _calculator.Calculate(signal);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.InRange(result.Score, 40, 80); // Conservative assumptions
+        Assert.Equal(6, result.Dimensions.Length); // All 6 dimensions
+        Assert.True(result.NeedsReview); // Low confidence triggers review
+    }
+
+    [Fact]
+    public void Calculate_WithHighRiskSignals_ReturnsHighScore()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 4, // R4: Reachable from entrypoint
+            IsInKev = true, // Known exploited
+            EpssProbability = 0.85,
+            VexStatus = "affected",
+            CveId = "CVE-2024-12345"
+        };
+
+        // Act
+        var result = _calculator.Calculate(signal);
+
+        // Assert
+        Assert.InRange(result.Score, 70, 100); // KEV floor should kick in
+        Assert.Equal("Critical", result.RiskTier);
+        Assert.Contains(result.AppliedGuardrails, g => g.StartsWith("kev_floor"));
+    }
+
+    [Fact]
+    public void Calculate_WithMitigatedSignals_ReturnsLowScore()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 0, // R0: Unreachable
+            BackportDetected = true,
+            BackportConfidence = 0.95,
+            VexStatus = "not_affected",
+            VexJustification = "Component not used in this deployment"
+        };
+
+        // Act
+        var result = _calculator.Calculate(signal);
+
+        // Assert
+        Assert.InRange(result.Score, 0, 25); // not_affected cap
+        Assert.Equal("Informational", result.RiskTier);
+    }
+
+    [Fact]
+    public void Calculate_AllDimensionsPopulated_ReturnsCorrectStructure()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 2,
+            CallGraphConfidence = 0.8,
+            InstrumentationCoverage = 0.7,
+            RuntimeInvocationCount = 50,
+            BackportDetected = false,
+            EpssProbability = 0.25,
+            CvssBaseScore = 7.5,
+            SbomCompleteness = 0.9,
+            SbomSigned = true,
+            VexStatus = "under_investigation",
+            CveId = "CVE-2024-99999",
+            Purl = "pkg:npm/example@1.0.0"
+        };
+
+        // Act
+        var result = _calculator.Calculate(signal);
+
+        // Assert
+        Assert.Equal(6, result.Dimensions.Length);
+
+        // Check each dimension is present
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.Reachability);
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.RuntimeSignals);
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.BackportEvidence);
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.Exploitability);
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.SourceConfidence);
+        Assert.Contains(result.Dimensions, d => d.Dimension == EwsDimension.MitigationStatus);
+
+        // Check metadata propagated
+        Assert.Equal("CVE-2024-99999", result.CveId);
+        Assert.Equal("pkg:npm/example@1.0.0", result.Purl);
+        Assert.Equal(_timeProvider.GetUtcNow(), result.CalculatedAt);
+    }
+
+    [Fact]
+    public void Calculate_CustomWeights_UsesProvidedWeights()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            IsInKev = false,
+            EpssProbability = 0.9 // High exploitability
+        };
+
+        var exploitHeavyWeights = new EwsDimensionWeights
+        {
+            Reachability = 0.1,
+            RuntimeSignals = 0.1,
+            BackportEvidence = 0.05,
+            Exploitability = 0.5, // Heavy weight on exploitability
+            SourceConfidence = 0.1,
+            MitigationStatus = 0.15
+        };
+
+        // Act
+        var result = _calculator.Calculate(signal, exploitHeavyWeights);
+
+        // Assert
+        var xplDim = result.GetDimension(EwsDimension.Exploitability);
+        Assert.NotNull(xplDim);
+        Assert.Equal(0.5, xplDim.Weight);
+    }
+
+    [Fact]
+    public void Calculate_SpeculativeScore_AppliesCap()
+    {
+        // Arrange - no real evidence, just defaults/assumptions
+        var signal = new EwsSignalInput
+        {
+            CveId = "CVE-2024-00001"
+        };
+
+        var guardrails = new EwsGuardrails
+        {
+            SpeculativeCap = 55
+        };
+
+        // Act
+        var result = _calculator.Calculate(signal, guardrails: guardrails);
+
+        // Assert
+        Assert.True(result.Score <= 55, $"Score {result.Score} should be capped at speculative cap 55");
+        Assert.True(result.NeedsReview); // Should need review due to low confidence
+    }
+
+    [Fact]
+    public void Calculate_Deterministic_SameInputsProduceSameOutput()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 3,
+            EpssProbability = 0.45,
+            VexStatus = "affected"
+        };
+
+        // Act
+        var result1 = _calculator.Calculate(signal);
+        var result2 = _calculator.Calculate(signal);
+
+        // Assert - should be identical
+        Assert.Equal(result1.Score, result2.Score);
+        Assert.Equal(result1.RawScore, result2.RawScore);
+        Assert.Equal(result1.Confidence, result2.Confidence);
+        Assert.Equal(result1.CalculatedAt, result2.CalculatedAt);
+    }
+
+    [Fact]
+    public void CalculateDimension_ReachabilityR4_ReturnsHighScore()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 4,
+            CallGraphConfidence = 0.9
+        };
+
+        // Act
+        var dimScore = _calculator.CalculateDimension(EwsDimension.Reachability, signal, 0.25);
+
+        // Assert
+        Assert.Equal(EwsDimension.Reachability, dimScore.Dimension);
+        Assert.Equal("RCH", dimScore.Code);
+        Assert.InRange(dimScore.Score, 90, 100);
+        Assert.True(dimScore.Confidence > 0.7);
+    }
+
+    [Fact]
+    public void CalculateDimension_ReachabilityR0_ReturnsLowScore()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            ReachabilityTier = 0,
+            CallGraphConfidence = 0.95
+        };
+
+        // Act
+        var dimScore = _calculator.CalculateDimension(EwsDimension.Reachability, signal, 0.25);
+
+        // Assert
+        Assert.InRange(dimScore.Score, 0, 10);
+    }
+
+    [Fact]
+    public void GetNormalizer_AllDimensions_ReturnsNonNull()
+    {
+        foreach (EwsDimension dimension in Enum.GetValues<EwsDimension>())
+        {
+            var normalizer = _calculator.GetNormalizer(dimension);
+            Assert.NotNull(normalizer);
+            Assert.Equal(dimension, normalizer.Dimension);
+        }
+    }
+}
+
+public sealed class EwsDimensionCodesTests
+{
+    [Theory]
+    [InlineData(EwsDimension.Reachability, "RCH")]
+    [InlineData(EwsDimension.RuntimeSignals, "RTS")]
+    [InlineData(EwsDimension.BackportEvidence, "BKP")]
+    [InlineData(EwsDimension.Exploitability, "XPL")]
+    [InlineData(EwsDimension.SourceConfidence, "SRC")]
+    [InlineData(EwsDimension.MitigationStatus, "MIT")]
+    public void ToCode_ReturnsCorrectCode(EwsDimension dimension, string expectedCode)
+    {
+        Assert.Equal(expectedCode, dimension.ToCode());
+    }
+
+    [Theory]
+    [InlineData("RCH", EwsDimension.Reachability)]
+    [InlineData("rch", EwsDimension.Reachability)]
+    [InlineData("XPL", EwsDimension.Exploitability)]
+    [InlineData("MIT", EwsDimension.MitigationStatus)]
+    public void FromCode_ReturnsCorrectDimension(string code, EwsDimension expected)
+    {
+        var result = EwsDimensionCodes.FromCode(code);
+        Assert.Equal(expected, result);
+    }
+
+    [Theory]
+    [InlineData("INVALID")]
+    [InlineData("")]
+    [InlineData(null)]
+    public void FromCode_InvalidCode_ReturnsNull(string? code)
+    {
+        var result = EwsDimensionCodes.FromCode(code!);
+        Assert.Null(result);
+    }
+}
+
+public sealed class EwsDimensionWeightsTests
+{
+    [Fact]
+    public void Default_IsNormalized()
+    {
+        var weights = EwsDimensionWeights.Default;
+        Assert.True(weights.IsNormalized());
+    }
+
+    [Fact]
+    public void Legacy_IsNormalized()
+    {
+        var weights = EwsDimensionWeights.Legacy;
+        Assert.True(weights.IsNormalized());
+    }
+
+    [Fact]
+    public void GetWeight_ReturnsCorrectWeights()
+    {
+        var weights = new EwsDimensionWeights
+        {
+            Reachability = 0.3,
+            RuntimeSignals = 0.1,
+            BackportEvidence = 0.1,
+            Exploitability = 0.25,
+            SourceConfidence = 0.1,
+            MitigationStatus = 0.15
+        };
+
+        Assert.Equal(0.3, weights.GetWeight(EwsDimension.Reachability));
+        Assert.Equal(0.25, weights.GetWeight(EwsDimension.Exploitability));
+        Assert.True(weights.IsNormalized());
+    }
+}
+
+public sealed class EwsGuardrailsTests
+{
+    [Fact]
+    public void Default_HasReasonableValues()
+    {
+        var guardrails = EwsGuardrails.Default;
+
+        Assert.InRange(guardrails.NotAffectedCap, 10, 50);
+        Assert.InRange(guardrails.RuntimeFloor, 20, 50);
+        Assert.InRange(guardrails.SpeculativeCap, 50, 70);
+        Assert.InRange(guardrails.KevFloor, 60, 90);
+        Assert.InRange(guardrails.BackportedCap, 10, 30);
+    }
+}
+
+public sealed class GuardrailsEngineTests
+{
+    private readonly GuardrailsEngine _engine = new();
+
+    [Fact]
+    public void Apply_KevFloor_RaisesScoreForKnownExploited()
+    {
+        // Arrange
+        var signal = new EwsSignalInput { IsInKev = true };
+        var guardrails = new EwsGuardrails { KevFloor = 70 };
+
+        // Act
+        var result = _engine.Apply(50, signal, [], guardrails);
+
+        // Assert
+        Assert.Equal(70, result.AdjustedScore);
+        Assert.Contains("kev_floor:70", result.AppliedGuardrails);
+    }
+
+    [Fact]
+    public void Apply_BackportedCap_LowersScoreForBackported()
+    {
+        // Arrange
+        var signal = new EwsSignalInput { BackportDetected = true };
+        var guardrails = new EwsGuardrails { BackportedCap = 20 };
+
+        // Act
+        var result = _engine.Apply(75, signal, [], guardrails);
+
+        // Assert
+        Assert.Equal(20, result.AdjustedScore);
+        Assert.Contains("backported_cap:20", result.AppliedGuardrails);
+    }
+
+    [Fact]
+    public void Apply_NotAffectedCap_LowersScoreForMitigated()
+    {
+        // Arrange
+        var signal = new EwsSignalInput { VexStatus = "not_affected" };
+        var guardrails = new EwsGuardrails { NotAffectedCap = 25 };
+
+        // Act
+        var result = _engine.Apply(60, signal, [], guardrails);
+
+        // Assert
+        Assert.Equal(25, result.AdjustedScore);
+        Assert.Contains("not_affected_cap:25", result.AppliedGuardrails);
+    }
+
+    [Fact]
+    public void Apply_NoGuardrailsTriggered_ReturnsOriginalScore()
+    {
+        // Arrange
+        var signal = new EwsSignalInput
+        {
+            VexStatus = "affected",
+            IsInKev = false,
+            BackportDetected = false
+        };
+        var guardrails = EwsGuardrails.Default;
+
+        // Act
+        var result = _engine.Apply(55, signal, [], guardrails);
+
+        // Assert
+        Assert.Equal(55, result.AdjustedScore);
+        Assert.Empty(result.AppliedGuardrails);
+        Assert.False(result.WasModified);
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/EwsNormalizerTests.cs

@@ -0,0 +1,286 @@
+// -----------------------------------------------------------------------------
+// EwsNormalizerTests.cs
+// Sprint: SPRINT_20260208_045_Policy_evidence_weighted_score_model
+// Task: T1 - Evidence-Weighted Score (EWS) Model (6-Dimension Scoring)
+// Description: Unit tests for individual dimension normalizers.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Determinization.Scoring.EvidenceWeightedScoring;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+public sealed class ReachabilityNormalizerTests
+{
+    private readonly ReachabilityNormalizer _normalizer = new();
+
+    [Theory]
+    [InlineData(0, 0, 10)]    // R0: Unreachable
+    [InlineData(1, 15, 30)]   // R1: In dependency
+    [InlineData(2, 35, 50)]   // R2: Imported not called
+    [InlineData(3, 60, 80)]   // R3: Called not entrypoint
+    [InlineData(4, 90, 100)]  // R4: Reachable
+    public void Normalize_ReachabilityTier_ReturnsExpectedRange(int tier, int minScore, int maxScore)
+    {
+        var signal = new EwsSignalInput { ReachabilityTier = tier };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, minScore, maxScore);
+    }
+
+    [Fact]
+    public void Normalize_RuntimeTraceConfirmed_BoostsScore()
+    {
+        var signalWithTrace = new EwsSignalInput
+        {
+            ReachabilityTier = 3,
+            RuntimeTraceConfirmed = true
+        };
+        var signalWithoutTrace = new EwsSignalInput
+        {
+            ReachabilityTier = 3
+        };
+
+        var scoreWith = _normalizer.Normalize(signalWithTrace);
+        var scoreWithout = _normalizer.Normalize(signalWithoutTrace);
+
+        Assert.True(scoreWith > scoreWithout);
+    }
+
+    [Fact]
+    public void GetConfidence_NoSignals_ReturnsLowConfidence()
+    {
+        var signal = EwsSignalInput.Empty;
+        var confidence = _normalizer.GetConfidence(signal);
+        Assert.True(confidence < 0.3);
+    }
+
+    [Fact]
+    public void GetExplanation_ReturnsNonEmptyString()
+    {
+        var signal = new EwsSignalInput { ReachabilityTier = 3 };
+        var explanation = _normalizer.GetExplanation(signal, 70);
+        Assert.False(string.IsNullOrWhiteSpace(explanation));
+    }
+}
+
+public sealed class ExploitabilityNormalizerTests
+{
+    private readonly ExploitabilityNormalizer _normalizer = new();
+
+    [Fact]
+    public void Normalize_InKev_ReturnsMaximumScore()
+    {
+        var signal = new EwsSignalInput { IsInKev = true };
+        var score = _normalizer.Normalize(signal);
+        Assert.Equal(100, score);
+    }
+
+    [Theory]
+    [InlineData(0.0, 0, 20)]
+    [InlineData(0.5, 40, 70)]
+    [InlineData(0.9, 70, 95)]
+    [InlineData(1.0, 85, 100)]
+    public void Normalize_EpssProbability_ScalesAppropriately(double epss, int minScore, int maxScore)
+    {
+        var signal = new EwsSignalInput { EpssProbability = epss };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, minScore, maxScore);
+    }
+
+    [Fact]
+    public void Normalize_ExploitKitAvailable_HighScore()
+    {
+        var signal = new EwsSignalInput { ExploitKitAvailable = true };
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 70);
+    }
+
+    [Fact]
+    public void GetConfidence_InKev_ReturnsMaximumConfidence()
+    {
+        var signal = new EwsSignalInput { IsInKev = true };
+        var confidence = _normalizer.GetConfidence(signal);
+        Assert.Equal(1.0, confidence);
+    }
+}
+
+public sealed class BackportEvidenceNormalizerTests
+{
+    private readonly BackportEvidenceNormalizer _normalizer = new();
+
+    [Fact]
+    public void Normalize_VendorBackportConfirmed_ReturnsVeryLowScore()
+    {
+        var signal = new EwsSignalInput { VendorBackportConfirmed = true };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, 0, 10);
+    }
+
+    [Fact]
+    public void Normalize_BackportDetectedWithHighConfidence_ReturnsLowScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            BackportDetected = true,
+            BackportConfidence = 0.9
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, 0, 15);
+    }
+
+    [Fact]
+    public void Normalize_NoBackportWithHighConfidence_ReturnsHighScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            BackportDetected = false,
+            BackportConfidence = 0.9
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 90);
+    }
+
+    [Fact]
+    public void Normalize_NoAnalysis_AssumesVulnerable()
+    {
+        var signal = EwsSignalInput.Empty;
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 70);
+    }
+}
+
+public sealed class MitigationStatusNormalizerTests
+{
+    private readonly MitigationStatusNormalizer _normalizer = new();
+
+    [Theory]
+    [InlineData("not_affected", 0, 10)]
+    [InlineData("fixed", 5, 15)]
+    [InlineData("under_investigation", 50, 70)]
+    [InlineData("affected", 85, 95)]
+    [InlineData("exploitable", 95, 100)]
+    public void Normalize_VexStatus_ReturnsExpectedRange(string status, int minScore, int maxScore)
+    {
+        var signal = new EwsSignalInput { VexStatus = status };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, minScore, maxScore);
+    }
+
+    [Fact]
+    public void Normalize_WorkaroundApplied_ReducesScore()
+    {
+        var signalWithWorkaround = new EwsSignalInput
+        {
+            VexStatus = "affected",
+            WorkaroundApplied = true
+        };
+        var signalWithoutWorkaround = new EwsSignalInput
+        {
+            VexStatus = "affected"
+        };
+
+        var scoreWith = _normalizer.Normalize(signalWithWorkaround);
+        var scoreWithout = _normalizer.Normalize(signalWithoutWorkaround);
+
+        Assert.True(scoreWith < scoreWithout);
+    }
+
+    [Fact]
+    public void Normalize_NetworkControlsApplied_ReducesScore()
+    {
+        var signalWithControls = new EwsSignalInput
+        {
+            VexStatus = "affected",
+            NetworkControlsApplied = true
+        };
+        var signalWithoutControls = new EwsSignalInput
+        {
+            VexStatus = "affected"
+        };
+
+        var scoreWith = _normalizer.Normalize(signalWithControls);
+        var scoreWithout = _normalizer.Normalize(signalWithoutControls);
+
+        Assert.True(scoreWith < scoreWithout);
+    }
+}
+
+public sealed class RuntimeSignalsNormalizerTests
+{
+    private readonly RuntimeSignalsNormalizer _normalizer = new();
+
+    [Fact]
+    public void Normalize_HighInstrumentationNoInvocations_LowScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            InstrumentationCoverage = 0.9,
+            RuntimeInvocationCount = 0,
+            ApmActiveUsage = false
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, 0, 30);
+    }
+
+    [Fact]
+    public void Normalize_HighInvocationCount_HighScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            RuntimeInvocationCount = 5000
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 70);
+    }
+
+    [Fact]
+    public void Normalize_ApmActiveUsage_HighScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            ApmActiveUsage = true
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 70);
+    }
+}
+
+public sealed class SourceConfidenceNormalizerTests
+{
+    private readonly SourceConfidenceNormalizer _normalizer = new();
+
+    [Fact]
+    public void Normalize_HighConfidenceSource_LowRiskScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            SbomCompleteness = 0.95,
+            SbomSigned = true,
+            AttestationCount = 3,
+            LineageVerified = true
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.InRange(score, 0, 20); // Low risk from source uncertainty
+    }
+
+    [Fact]
+    public void Normalize_LowConfidenceSource_HighRiskScore()
+    {
+        var signal = new EwsSignalInput
+        {
+            SbomCompleteness = 0.3,
+            SbomSigned = false,
+            AttestationCount = 0,
+            LineageVerified = false
+        };
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 60); // High risk from source uncertainty
+    }
+
+    [Fact]
+    public void Normalize_NoSignals_AssumesHighUncertainty()
+    {
+        var signal = EwsSignalInput.Empty;
+        var score = _normalizer.Normalize(signal);
+        Assert.True(score >= 70);
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/ImpactScoreCalculatorTests.cs

@@ -0,0 +1,260 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using StellaOps.Policy.Determinization.Scoring;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+public class ImpactScoreCalculatorTests
+{
+    private readonly ImpactScoreCalculator _calculator;
+
+    public ImpactScoreCalculatorTests()
+    {
+        _calculator = new ImpactScoreCalculator(NullLogger<ImpactScoreCalculator>.Instance);
+    }
+
+    #region Calculate Tests
+
+    [Fact]
+    public void Calculate_ProductionHighSensitivityCriticalSla_ReturnsHighScore()
+    {
+        // Arrange
+        var context = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Healthcare,
+            FleetPrevalence = 0.9,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 9.8
+        };
+
+        // Act
+        var score = _calculator.Calculate(context);
+
+        // Assert
+        score.Score.Should().BeGreaterThan(0.8);
+        score.BasisPoints.Should().BeGreaterThan(8000);
+        score.EnvironmentExposure.Should().Be(1.0);
+        score.CvssSeverityScore.Should().BeApproximately(0.98, 0.01);
+    }
+
+    [Fact]
+    public void Calculate_DevelopmentLowSensitivity_ReturnsLowScore()
+    {
+        // Arrange
+        var context = new ImpactContext
+        {
+            Environment = EnvironmentType.Development,
+            DataSensitivity = DataSensitivity.Public,
+            FleetPrevalence = 0.1,
+            SlaTier = SlaTier.NonCritical,
+            CvssScore = 2.0
+        };
+
+        // Act
+        var score = _calculator.Calculate(context);
+
+        // Assert
+        score.Score.Should().BeLessThan(0.2);
+        score.BasisPoints.Should().BeLessThan(2000);
+        score.EnvironmentExposure.Should().Be(0.0);
+        score.DataSensitivityScore.Should().Be(0.0);
+    }
+
+    [Fact]
+    public void Calculate_DefaultForUnknowns_ReturnsModerateScore()
+    {
+        // Arrange
+        var context = ImpactContext.DefaultForUnknowns();
+
+        // Act
+        var score = _calculator.Calculate(context);
+
+        // Assert - default context assumes production, internal data, 0.5 fleet, standard SLA, CVSS 5.0
+        score.Score.Should().BeInRange(0.3, 0.6);
+    }
+
+    [Fact]
+    public void Calculate_CustomWeights_UsesProvidedWeights()
+    {
+        // Arrange
+        var context = new ImpactContext
+        {
+            Environment = EnvironmentType.Production,
+            DataSensitivity = DataSensitivity.Classified,
+            FleetPrevalence = 1.0,
+            SlaTier = SlaTier.MissionCritical,
+            CvssScore = 10.0
+        };
+
+        // All weights on CVSS, should return 1.0
+        var weights = new ImpactFactorWeights
+        {
+            EnvironmentExposureWeight = 0.0,
+            DataSensitivityWeight = 0.0,
+            FleetPrevalenceWeight = 0.0,
+            SlaTierWeight = 0.0,
+            CvssSeverityWeight = 1.0
+        };
+
+        // Act
+        var score = _calculator.Calculate(context, weights);
+
+        // Assert
+        score.Score.Should().BeApproximately(1.0, 0.001);
+    }
+
+    [Fact]
+    public void Calculate_IsDeterministic_SameInputSameOutput()
+    {
+        // Arrange
+        var context = new ImpactContext
+        {
+            Environment = EnvironmentType.Staging,
+            DataSensitivity = DataSensitivity.Pii,
+            FleetPrevalence = 0.5,
+            SlaTier = SlaTier.Important,
+            CvssScore = 7.5
+        };
+
+        // Act
+        var score1 = _calculator.Calculate(context);
+        var score2 = _calculator.Calculate(context);
+
+        // Assert
+        score1.Score.Should().Be(score2.Score);
+        score1.BasisPoints.Should().Be(score2.BasisPoints);
+    }
+
+    #endregion
+
+    #region NormalizeEnvironment Tests
+
+    [Theory]
+    [InlineData(EnvironmentType.Development, 0.0)]
+    [InlineData(EnvironmentType.Testing, 0.33)]
+    [InlineData(EnvironmentType.Staging, 0.66)]
+    [InlineData(EnvironmentType.Production, 1.0)]
+    public void NormalizeEnvironment_ReturnsExpectedScore(EnvironmentType env, double expected)
+    {
+        // Act
+        var score = _calculator.NormalizeEnvironment(env);
+
+        // Assert
+        score.Should().BeApproximately(expected, 0.01);
+    }
+
+    #endregion
+
+    #region NormalizeDataSensitivity Tests
+
+    [Theory]
+    [InlineData(DataSensitivity.Public, 0.0)]
+    [InlineData(DataSensitivity.Internal, 0.2)]
+    [InlineData(DataSensitivity.Pii, 0.5)]
+    [InlineData(DataSensitivity.Financial, 0.7)]
+    [InlineData(DataSensitivity.Healthcare, 0.8)]
+    [InlineData(DataSensitivity.Classified, 1.0)]
+    public void NormalizeDataSensitivity_ReturnsExpectedScore(DataSensitivity sensitivity, double expected)
+    {
+        // Act
+        var score = _calculator.NormalizeDataSensitivity(sensitivity);
+
+        // Assert
+        score.Should().BeApproximately(expected, 0.01);
+    }
+
+    #endregion
+
+    #region NormalizeSlaTier Tests
+
+    [Theory]
+    [InlineData(SlaTier.NonCritical, 0.0)]
+    [InlineData(SlaTier.Standard, 0.25)]
+    [InlineData(SlaTier.Important, 0.5)]
+    [InlineData(SlaTier.Critical, 0.75)]
+    [InlineData(SlaTier.MissionCritical, 1.0)]
+    public void NormalizeSlaTier_ReturnsExpectedScore(SlaTier tier, double expected)
+    {
+        // Act
+        var score = _calculator.NormalizeSlaTier(tier);
+
+        // Assert
+        score.Should().BeApproximately(expected, 0.01);
+    }
+
+    #endregion
+
+    #region NormalizeCvss Tests
+
+    [Theory]
+    [InlineData(0.0, 0.0)]
+    [InlineData(5.0, 0.5)]
+    [InlineData(10.0, 1.0)]
+    [InlineData(-1.0, 0.0)] // Clamped
+    [InlineData(15.0, 1.0)] // Clamped
+    public void NormalizeCvss_ReturnsExpectedScore(double cvss, double expected)
+    {
+        // Act
+        var score = _calculator.NormalizeCvss(cvss);
+
+        // Assert
+        score.Should().BeApproximately(expected, 0.01);
+    }
+
+    #endregion
+
+    #region ImpactFactorWeights Tests
+
+    [Fact]
+    public void ImpactFactorWeights_Default_IsNormalized()
+    {
+        // Act & Assert
+        ImpactFactorWeights.Default.IsNormalized().Should().BeTrue();
+        ImpactFactorWeights.Default.TotalWeight.Should().BeApproximately(1.0, 0.001);
+    }
+
+    [Fact]
+    public void ImpactFactorWeights_Custom_TotalWeightCalculated()
+    {
+        // Arrange
+        var weights = new ImpactFactorWeights
+        {
+            EnvironmentExposureWeight = 0.1,
+            DataSensitivityWeight = 0.2,
+            FleetPrevalenceWeight = 0.3,
+            SlaTierWeight = 0.15,
+            CvssSeverityWeight = 0.25
+        };
+
+        // Act & Assert
+        weights.TotalWeight.Should().BeApproximately(1.0, 0.001);
+        weights.IsNormalized().Should().BeTrue();
+    }
+
+    #endregion
+
+    #region ImpactScore Tests
+
+    [Fact]
+    public void ImpactScore_Create_CalculatesBasisPointsCorrectly()
+    {
+        // Arrange & Act
+        var score = ImpactScore.Create(
+            envExposure: 1.0,
+            dataSensitivity: 0.5,
+            fleetPrevalence: 0.5,
+            slaTier: 0.5,
+            cvssSeverity: 0.5,
+            ImpactFactorWeights.Default,
+            DateTimeOffset.UtcNow);
+
+        // Assert
+        // Score = 1.0*0.2 + 0.5*0.2 + 0.5*0.15 + 0.5*0.15 + 0.5*0.3 = 0.2 + 0.1 + 0.075 + 0.075 + 0.15 = 0.6
+        score.Score.Should().BeApproximately(0.6, 0.01);
+        score.BasisPoints.Should().Be(6000);
+    }
+
+    #endregion
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/Triage/TriageQueueEvaluatorTests.cs

@@ -0,0 +1,357 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring.Triage;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring.Triage;
+
+public sealed class TriageQueueEvaluatorTests
+{
+    private static readonly DateTimeOffset ReferenceTime = new(2026, 2, 8, 12, 0, 0, TimeSpan.Zero);
+
+    private readonly TriageQueueEvaluator _evaluator;
+    private readonly TriageQueueOptions _options;
+
+    public TriageQueueEvaluatorTests()
+    {
+        _options = new TriageQueueOptions();
+        _evaluator = new TriageQueueEvaluator(
+            NullLogger<TriageQueueEvaluator>.Instance,
+            Options.Create(_options));
+    }
+
+    #region EvaluateSingle Tests
+
+    [Fact]
+    public void EvaluateSingle_FreshObservation_ReturnsNull()
+    {
+        var obs = CreateObservation(ageDays: 0);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().BeNull("fresh observation should not be queued");
+    }
+
+    [Fact]
+    public void EvaluateSingle_SlightlyAged_ReturnsNull()
+    {
+        // 5 days old with 14-day half-life => multiplier ≈ 0.78, above approaching threshold 0.70
+        var obs = CreateObservation(ageDays: 5);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().BeNull("multiplier 0.78 is above approaching threshold 0.70");
+    }
+
+    [Fact]
+    public void EvaluateSingle_ApproachingStaleness_ReturnsLowPriority()
+    {
+        // 8 days old with 14-day half-life => multiplier ≈ 0.67, between 0.50 and 0.70
+        var obs = CreateObservation(ageDays: 8);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.Priority.Should().Be(TriagePriority.Low);
+        result.CurrentMultiplier.Should().BeApproximately(0.67, 0.05);
+    }
+
+    [Fact]
+    public void EvaluateSingle_Stale_ReturnsMediumPriority()
+    {
+        // 14 days old (one half-life) => multiplier = 0.50, at staleness threshold
+        // Actually 15 days => multiplier ≈ 0.48, below 0.50 => Medium
+        var obs = CreateObservation(ageDays: 15);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.Priority.Should().Be(TriagePriority.Medium);
+        result.DaysUntilStale.Should().BeNegative("already stale");
+    }
+
+    [Fact]
+    public void EvaluateSingle_HeavilyDecayed_ReturnsHighPriority()
+    {
+        // 28 days (two half-lives) => multiplier ≈ 0.25
+        var obs = CreateObservation(ageDays: 28);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.Priority.Should().Be(TriagePriority.High);
+    }
+
+    [Fact]
+    public void EvaluateSingle_AtFloor_ReturnsCriticalPriority()
+    {
+        // 100 days => multiplier at floor (0.35 but compare to 0.10 threshold)
+        // With floor=0.35, actual multiplier can't go below 0.35
+        // Need floor < CriticalThreshold to get Critical
+        // Use custom decay with floor=0.05
+        var decay = ObservationDecay.WithSettings(
+            ReferenceTime.AddDays(-200),
+            ReferenceTime.AddDays(-200),
+            halfLifeDays: 14.0,
+            floor: 0.05,
+            stalenessThreshold: 0.50);
+        var obs = new TriageObservation
+        {
+            Cve = "CVE-2026-9999",
+            Purl = "pkg:npm/test@1.0.0",
+            TenantId = "tenant-1",
+            Decay = decay
+        };
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.Priority.Should().Be(TriagePriority.Critical);
+    }
+
+    [Fact]
+    public void EvaluateSingle_PreservesCveAndPurl()
+    {
+        var obs = CreateObservation(ageDays: 20, cve: "CVE-2026-1234", purl: "pkg:maven/org.example/lib@2.0");
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.Cve.Should().Be("CVE-2026-1234");
+        result.Purl.Should().Be("pkg:maven/org.example/lib@2.0");
+    }
+
+    [Fact]
+    public void EvaluateSingle_SetsEvaluatedAt()
+    {
+        var obs = CreateObservation(ageDays: 20);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.EvaluatedAt.Should().Be(ReferenceTime);
+    }
+
+    [Fact]
+    public void EvaluateSingle_WithSignalGaps_SetsRecommendedAction()
+    {
+        var gaps = new List<SignalGap>
+        {
+            new() { Signal = "EPSS", Reason = SignalGapReason.NotQueried, Weight = 0.20 },
+            new() { Signal = "VEX", Reason = SignalGapReason.NotAvailable, Weight = 0.30 }
+        };
+        var obs = CreateObservation(ageDays: 20, gaps: gaps);
+
+        var result = _evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().NotBeNull();
+        result!.RecommendedAction.Should().Contain("EPSS");
+        result.RecommendedAction.Should().Contain("VEX");
+        result.SignalGaps.Should().HaveCount(2);
+    }
+
+    [Fact]
+    public void EvaluateSingle_ApproachingDisabled_ReturnsNull()
+    {
+        var options = new TriageQueueOptions { IncludeApproaching = false };
+        var evaluator = new TriageQueueEvaluator(
+            NullLogger<TriageQueueEvaluator>.Instance,
+            Options.Create(options));
+
+        var obs = CreateObservation(ageDays: 8); // approaching but not stale
+
+        var result = evaluator.EvaluateSingle(obs, ReferenceTime);
+
+        result.Should().BeNull("approaching items should be excluded when IncludeApproaching=false");
+    }
+
+    #endregion
+
+    #region EvaluateAsync Tests
+
+    [Fact]
+    public async Task EvaluateAsync_EmptyList_ReturnsEmptySnapshot()
+    {
+        var snapshot = await _evaluator.EvaluateAsync([], ReferenceTime);
+
+        snapshot.Items.Should().BeEmpty();
+        snapshot.TotalEvaluated.Should().Be(0);
+        snapshot.StaleCount.Should().Be(0);
+        snapshot.ApproachingCount.Should().Be(0);
+        snapshot.EvaluatedAt.Should().Be(ReferenceTime);
+    }
+
+    [Fact]
+    public async Task EvaluateAsync_MixedObservations_SortsByPriorityThenUrgency()
+    {
+        var observations = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 8, cve: "CVE-A"),   // Low (approaching)
+            CreateObservation(ageDays: 20, cve: "CVE-B"),  // Medium (stale)
+            CreateObservation(ageDays: 30, cve: "CVE-C"),  // High (heavily decayed)
+            CreateObservation(ageDays: 2, cve: "CVE-D"),   // None (fresh)
+        };
+
+        var snapshot = await _evaluator.EvaluateAsync(observations, ReferenceTime);
+
+        snapshot.TotalEvaluated.Should().Be(4);
+        snapshot.Items.Should().HaveCount(3, "fresh observation should be excluded");
+        snapshot.Items[0].Priority.Should().Be(TriagePriority.High, "highest priority first");
+        snapshot.Items[1].Priority.Should().Be(TriagePriority.Medium);
+        snapshot.Items[2].Priority.Should().Be(TriagePriority.Low);
+    }
+
+    [Fact]
+    public async Task EvaluateAsync_SamePriority_SortsByDaysUntilStale()
+    {
+        var observations = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 16, cve: "CVE-X"),  // Medium, more stale
+            CreateObservation(ageDays: 15, cve: "CVE-Y"),  // Medium, less stale
+        };
+
+        var snapshot = await _evaluator.EvaluateAsync(observations, ReferenceTime);
+
+        snapshot.Items.Should().HaveCount(2);
+        // Both Medium, sorted by daysUntilStale ascending (most negative first)
+        snapshot.Items[0].DaysUntilStale.Should().BeLessThan(snapshot.Items[1].DaysUntilStale);
+    }
+
+    [Fact]
+    public async Task EvaluateAsync_PrioritySummary_IsCorrect()
+    {
+        var observations = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 8, cve: "CVE-1"),   // Low
+            CreateObservation(ageDays: 9, cve: "CVE-2"),   // Low
+            CreateObservation(ageDays: 20, cve: "CVE-3"),  // Medium
+        };
+
+        var snapshot = await _evaluator.EvaluateAsync(observations, ReferenceTime);
+
+        snapshot.PrioritySummary.Should().ContainKey(TriagePriority.Low);
+        snapshot.PrioritySummary[TriagePriority.Low].Should().Be(2);
+        snapshot.PrioritySummary.Should().ContainKey(TriagePriority.Medium);
+        snapshot.PrioritySummary[TriagePriority.Medium].Should().Be(1);
+    }
+
+    [Fact]
+    public async Task EvaluateAsync_RespectsMaxSnapshotItems()
+    {
+        var options = new TriageQueueOptions { MaxSnapshotItems = 2 };
+        var evaluator = new TriageQueueEvaluator(
+            NullLogger<TriageQueueEvaluator>.Instance,
+            Options.Create(options));
+
+        var observations = Enumerable.Range(0, 10)
+            .Select(i => CreateObservation(ageDays: 15 + i, cve: $"CVE-{i:D4}"))
+            .ToList();
+
+        var snapshot = await evaluator.EvaluateAsync(observations, ReferenceTime);
+
+        snapshot.Items.Should().HaveCount(2);
+        snapshot.TotalEvaluated.Should().Be(10);
+    }
+
+    [Fact]
+    public async Task EvaluateAsync_Deterministic_SameInputsSameOutput()
+    {
+        var observations = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 10, cve: "CVE-A"),
+            CreateObservation(ageDays: 20, cve: "CVE-B"),
+            CreateObservation(ageDays: 30, cve: "CVE-C"),
+        };
+
+        var snapshot1 = await _evaluator.EvaluateAsync(observations, ReferenceTime);
+        var snapshot2 = await _evaluator.EvaluateAsync(observations, ReferenceTime);
+
+        snapshot1.Items.Count.Should().Be(snapshot2.Items.Count);
+        for (var i = 0; i < snapshot1.Items.Count; i++)
+        {
+            snapshot1.Items[i].Cve.Should().Be(snapshot2.Items[i].Cve);
+            snapshot1.Items[i].Priority.Should().Be(snapshot2.Items[i].Priority);
+            snapshot1.Items[i].CurrentMultiplier.Should().Be(snapshot2.Items[i].CurrentMultiplier);
+            snapshot1.Items[i].DaysUntilStale.Should().Be(snapshot2.Items[i].DaysUntilStale);
+        }
+    }
+
+    #endregion
+
+    #region ClassifyPriority Tests
+
+    [Theory]
+    [InlineData(0.95, TriagePriority.None)]
+    [InlineData(0.80, TriagePriority.None)]
+    [InlineData(0.65, TriagePriority.Low)]
+    [InlineData(0.55, TriagePriority.Low)]
+    [InlineData(0.45, TriagePriority.Medium)]
+    [InlineData(0.25, TriagePriority.High)]
+    [InlineData(0.08, TriagePriority.Critical)]
+    [InlineData(0.00, TriagePriority.Critical)]
+    public void ClassifyPriority_ReturnsExpectedTier(double multiplier, TriagePriority expected)
+    {
+        var result = _evaluator.ClassifyPriority(multiplier, stalenessThreshold: 0.50);
+
+        result.Should().Be(expected);
+    }
+
+    #endregion
+
+    #region CalculateDaysUntilStale Tests
+
+    [Fact]
+    public void CalculateDaysUntilStale_FreshObservation_ReturnsPositive()
+    {
+        var refreshedAt = ReferenceTime;
+        var result = TriageQueueEvaluator.CalculateDaysUntilStale(
+            refreshedAt, halfLifeDays: 14.0, stalenessThreshold: 0.50, floor: 0.35, ReferenceTime);
+
+        result.Should().BeApproximately(14.0, 0.1, "one half-life until 0.50 threshold");
+    }
+
+    [Fact]
+    public void CalculateDaysUntilStale_AlreadyStale_ReturnsNegative()
+    {
+        var refreshedAt = ReferenceTime.AddDays(-20);
+        var result = TriageQueueEvaluator.CalculateDaysUntilStale(
+            refreshedAt, halfLifeDays: 14.0, stalenessThreshold: 0.50, floor: 0.35, ReferenceTime);
+
+        result.Should().BeNegative("observation is past staleness threshold");
+    }
+
+    [Fact]
+    public void CalculateDaysUntilStale_FloorAboveThreshold_ReturnsMaxValue()
+    {
+        var result = TriageQueueEvaluator.CalculateDaysUntilStale(
+            ReferenceTime, halfLifeDays: 14.0, stalenessThreshold: 0.30, floor: 0.50, ReferenceTime);
+
+        result.Should().Be(double.MaxValue, "floor prevents reaching threshold");
+    }
+
+    #endregion
+
+    #region Helper Methods
+
+    private static TriageObservation CreateObservation(
+        double ageDays,
+        string cve = "CVE-2026-0001",
+        string purl = "pkg:npm/test@1.0.0",
+        string tenantId = "tenant-1",
+        IReadOnlyList<SignalGap>? gaps = null)
+    {
+        var refreshedAt = ReferenceTime.AddDays(-ageDays);
+        return new TriageObservation
+        {
+            Cve = cve,
+            Purl = purl,
+            TenantId = tenantId,
+            Decay = ObservationDecay.Create(refreshedAt, refreshedAt),
+            SignalGaps = gaps ?? []
+        };
+    }
+
+    #endregion
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/Triage/UnknownTriageQueueServiceTests.cs

@@ -0,0 +1,228 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using Moq;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring.Triage;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring.Triage;
+
+public sealed class UnknownTriageQueueServiceTests
+{
+    private static readonly DateTimeOffset ReferenceTime = new(2026, 2, 8, 12, 0, 0, TimeSpan.Zero);
+
+    private readonly TriageQueueOptions _options = new();
+    private readonly TriageQueueEvaluator _evaluator;
+    private readonly Mock<ITriageObservationSource> _sourceMock;
+    private readonly InMemoryTriageReanalysisSink _sink;
+    private readonly UnknownTriageQueueService _service;
+
+    public UnknownTriageQueueServiceTests()
+    {
+        _evaluator = new TriageQueueEvaluator(
+            NullLogger<TriageQueueEvaluator>.Instance,
+            Options.Create(_options));
+
+        _sourceMock = new Mock<ITriageObservationSource>();
+        _sink = new InMemoryTriageReanalysisSink(NullLogger<InMemoryTriageReanalysisSink>.Instance);
+
+        var fakeTimeProvider = new FakeTimeProvider(ReferenceTime);
+
+        _service = new UnknownTriageQueueService(
+            _evaluator,
+            _sourceMock.Object,
+            _sink,
+            NullLogger<UnknownTriageQueueService>.Instance,
+            Options.Create(_options),
+            fakeTimeProvider);
+    }
+
+    [Fact]
+    public async Task ExecuteCycleAsync_NoCandidates_ReturnsEmptySnapshot()
+    {
+        _sourceMock
+            .Setup(s => s.GetCandidatesAsync(null, It.IsAny<int>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync([]);
+
+        var snapshot = await _service.ExecuteCycleAsync();
+
+        snapshot.Items.Should().BeEmpty();
+        snapshot.TotalEvaluated.Should().Be(0);
+        _sink.Count.Should().Be(0);
+    }
+
+    [Fact]
+    public async Task ExecuteCycleAsync_StaleObservations_EnqueuedToSink()
+    {
+        var candidates = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 20, cve: "CVE-STALE-1"),  // Medium
+            CreateObservation(ageDays: 30, cve: "CVE-STALE-2"),  // High
+        };
+
+        _sourceMock
+            .Setup(s => s.GetCandidatesAsync(null, It.IsAny<int>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(candidates);
+
+        var snapshot = await _service.ExecuteCycleAsync();
+
+        snapshot.Items.Should().HaveCount(2);
+        _sink.Count.Should().Be(2, "both stale items should be enqueued");
+    }
+
+    [Fact]
+    public async Task ExecuteCycleAsync_OnlyApproaching_NotEnqueued()
+    {
+        var candidates = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 8, cve: "CVE-APPROACH"),  // Low priority
+        };
+
+        _sourceMock
+            .Setup(s => s.GetCandidatesAsync(null, It.IsAny<int>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(candidates);
+
+        var snapshot = await _service.ExecuteCycleAsync();
+
+        snapshot.Items.Should().HaveCount(1, "approaching item is in snapshot");
+        _sink.Count.Should().Be(0, "approaching items are not enqueued for re-analysis");
+    }
+
+    [Fact]
+    public async Task ExecuteCycleAsync_MixedStaleAndFresh_OnlyStaleEnqueued()
+    {
+        var candidates = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 2, cve: "CVE-FRESH"),    // None
+            CreateObservation(ageDays: 8, cve: "CVE-APPROACH"), // Low
+            CreateObservation(ageDays: 20, cve: "CVE-STALE"),   // Medium
+        };
+
+        _sourceMock
+            .Setup(s => s.GetCandidatesAsync(null, It.IsAny<int>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(candidates);
+
+        var snapshot = await _service.ExecuteCycleAsync();
+
+        snapshot.TotalEvaluated.Should().Be(3);
+        _sink.Count.Should().Be(1, "only medium+ items are enqueued");
+
+        var enqueued = _sink.DrainAll();
+        enqueued[0].Cve.Should().Be("CVE-STALE");
+    }
+
+    [Fact]
+    public async Task ExecuteCycleAsync_WithTenantFilter_PassesToSource()
+    {
+        _sourceMock
+            .Setup(s => s.GetCandidatesAsync("tenant-42", It.IsAny<int>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync([]);
+
+        await _service.ExecuteCycleAsync(tenantId: "tenant-42");
+
+        _sourceMock.Verify(
+            s => s.GetCandidatesAsync("tenant-42", It.IsAny<int>(), It.IsAny<CancellationToken>()),
+            Times.Once);
+    }
+
+    [Fact]
+    public async Task EvaluateOnDemandAsync_DoesNotEnqueue()
+    {
+        var observations = new List<TriageObservation>
+        {
+            CreateObservation(ageDays: 20, cve: "CVE-DEMAND"),
+        };
+
+        var snapshot = await _service.EvaluateOnDemandAsync(observations, ReferenceTime);
+
+        snapshot.Items.Should().HaveCount(1);
+        _sink.Count.Should().Be(0, "on-demand evaluation should not auto-enqueue");
+    }
+
+    #region InMemoryTriageReanalysisSink Tests
+
+    [Fact]
+    public async Task InMemorySink_EnqueueAndDrain()
+    {
+        var items = new List<TriageItem>
+        {
+            CreateTriageItem("CVE-1", TriagePriority.Medium),
+            CreateTriageItem("CVE-2", TriagePriority.High),
+        };
+
+        var enqueued = await _sink.EnqueueAsync(items);
+
+        enqueued.Should().Be(2);
+        _sink.Count.Should().Be(2);
+
+        var drained = _sink.DrainAll();
+        drained.Should().HaveCount(2);
+        _sink.Count.Should().Be(0);
+    }
+
+    [Fact]
+    public void InMemorySink_TryDequeue_EmptyQueue_ReturnsFalse()
+    {
+        var result = _sink.TryDequeue(out var item);
+
+        result.Should().BeFalse();
+        item.Should().BeNull();
+    }
+
+    [Fact]
+    public async Task InMemorySink_PeekAll_DoesNotRemove()
+    {
+        await _sink.EnqueueAsync([CreateTriageItem("CVE-PEEK", TriagePriority.Critical)]);
+
+        var peeked = _sink.PeekAll();
+        peeked.Should().HaveCount(1);
+        _sink.Count.Should().Be(1, "peek should not remove items");
+    }
+
+    #endregion
+
+    #region Helpers
+
+    private static TriageObservation CreateObservation(double ageDays, string cve = "CVE-2026-0001")
+    {
+        var refreshedAt = ReferenceTime.AddDays(-ageDays);
+        return new TriageObservation
+        {
+            Cve = cve,
+            Purl = "pkg:npm/test@1.0.0",
+            TenantId = "tenant-1",
+            Decay = ObservationDecay.Create(refreshedAt, refreshedAt),
+        };
+    }
+
+    private static TriageItem CreateTriageItem(string cve, TriagePriority priority) => new()
+    {
+        Cve = cve,
+        Purl = "pkg:npm/test@1.0.0",
+        TenantId = "tenant-1",
+        Decay = ObservationDecay.Fresh(ReferenceTime),
+        CurrentMultiplier = 0.5,
+        Priority = priority,
+        AgeDays = 10,
+        DaysUntilStale = -5,
+        EvaluatedAt = ReferenceTime,
+    };
+
+    /// <summary>
+    /// Fake TimeProvider for deterministic testing.
+    /// </summary>
+    private sealed class FakeTimeProvider : TimeProvider
+    {
+        private readonly DateTimeOffset _now;
+
+        public FakeTimeProvider(DateTimeOffset now)
+        {
+            _now = now;
+        }
+
+        public override DateTimeOffset GetUtcNow() => _now;
+    }
+
+    #endregion
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/TrustScoreAlgebraFacadeTests.cs

@@ -0,0 +1,380 @@
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Time.Testing;
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring;
+using StellaOps.Policy.Scoring;
+using StellaOps.Policy.TrustLattice;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring;
+
+/// <summary>
+/// Unit tests for TrustScoreAlgebraFacade.
+/// Verifies the unified scoring pipeline produces deterministic, attestation-ready results.
+/// </summary>
+public sealed class TrustScoreAlgebraFacadeTests
+{
+    private readonly FakeTimeProvider _timeProvider = new(new DateTimeOffset(2026, 1, 15, 12, 0, 0, TimeSpan.Zero));
+
+    private TrustScoreAlgebraFacade CreateFacade()
+    {
+        var aggregator = new TrustScoreAggregator(NullLogger<TrustScoreAggregator>.Instance);
+        var uncertaintyCalculator = new UncertaintyScoreCalculator();
+        return new TrustScoreAlgebraFacade(
+            aggregator,
+            uncertaintyCalculator,
+            NullLogger<TrustScoreAlgebraFacade>.Instance,
+            _timeProvider);
+    }
+
+    #region Basic Computation Tests
+
+    [Fact]
+    public void ComputeTrustScore_EmptySignals_ReturnsValidPredicate()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234"
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.Should().NotBeNull();
+        result.Predicate.ArtifactId.Should().Be("pkg:maven/test@1.0");
+        result.Predicate.VulnerabilityId.Should().Be("CVE-2024-1234");
+        result.Predicate.TrustScoreBps.Should().BeInRange(0, 10000);
+        result.Predicate.LatticeVerdict.Should().Be(K4Value.Unknown);
+    }
+
+    [Fact]
+    public void ComputeTrustScore_WithSignals_ReturnsCalculatedScore()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var signals = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow())
+            with
+            {
+                Reachability = SignalState<ReachabilityEvidence>.Present(
+                    new ReachabilityEvidence(ReachabilityStatus.Reachable, 0, 0, null)),
+                Vex = SignalState<VexClaimSummary>.Present(
+                    new VexClaimSummary("affected", null, null, null, null, null))
+            };
+
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234",
+            Signals = signals
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.Dimensions.ReachabilityBps.Should().Be(10000); // Reachable = max score
+        result.Predicate.Dimensions.VexBps.Should().Be(10000); // Affected = max risk
+    }
+
+    [Fact]
+    public void ComputeTrustScore_UnreachableVulnerability_LowerScore()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var signals = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow())
+            with
+            {
+                Reachability = SignalState<ReachabilityEvidence>.Present(
+                    new ReachabilityEvidence(ReachabilityStatus.Unreachable, 0, 0, null)),
+                Vex = SignalState<VexClaimSummary>.Present(
+                    new VexClaimSummary("affected", null, null, null, null, null))
+            };
+
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234",
+            Signals = signals
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.Dimensions.ReachabilityBps.Should().Be(0); // Unreachable = no risk from reachability
+    }
+
+    #endregion
+
+    #region K4 Lattice Tests
+
+    [Fact]
+    public void ComputeTrustScore_ConflictingSignals_ReturnsConflict()
+    {
+        // Arrange: VEX says not_affected, EPSS says high probability
+        var facade = CreateFacade();
+        var signals = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow())
+            with
+            {
+                Vex = SignalState<VexClaimSummary>.Present(
+                    new VexClaimSummary("not_affected", null, null, null, null, null)),
+                Epss = SignalState<EpssEvidence>.Present(
+                    new EpssEvidence(0.85, 0.95)) // High EPSS = True in K4
+            };
+
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234",
+            Signals = signals
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.LatticeVerdict.Should().Be(K4Value.Conflict);
+    }
+
+    [Fact]
+    public void ComputeTrustScore_AllTrueSignals_ReturnsTrueVerdict()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var signals = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow())
+            with
+            {
+                Vex = SignalState<VexClaimSummary>.Present(
+                    new VexClaimSummary("affected", null, null, null, null, null)),
+                Reachability = SignalState<ReachabilityEvidence>.Present(
+                    new ReachabilityEvidence(ReachabilityStatus.Reachable, 0, 0, null)),
+                Epss = SignalState<EpssEvidence>.Present(
+                    new EpssEvidence(0.75, 0.90))
+            };
+
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            Signals = signals
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.LatticeVerdict.Should().Be(K4Value.True);
+    }
+
+    #endregion
+
+    #region Score.v1 Predicate Format Tests
+
+    [Fact]
+    public void ComputeTrustScore_ReturnsCorrectPredicateType()
+    {
+        // Assert
+        ScoreV1Predicate.PredicateType.Should().Be("https://stella-ops.org/predicates/score/v1");
+    }
+
+    [Fact]
+    public void ComputeTrustScore_IncludesAllRequiredFields()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234",
+            TenantId = "tenant-123"
+        };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        result.Predicate.ArtifactId.Should().NotBeNullOrEmpty();
+        result.Predicate.TrustScoreBps.Should().BeInRange(0, 10000);
+        result.Predicate.Tier.Should().NotBeNullOrEmpty();
+        result.Predicate.UncertaintyBps.Should().BeInRange(0, 10000);
+        result.Predicate.Dimensions.Should().NotBeNull();
+        result.Predicate.WeightsUsed.Should().NotBeNull();
+        result.Predicate.PolicyDigest.Should().NotBeNullOrEmpty();
+        result.Predicate.ComputedAt.Should().Be(_timeProvider.GetUtcNow());
+        result.Predicate.TenantId.Should().Be("tenant-123");
+    }
+
+    [Fact]
+    public void ComputeTrustScore_PolicyDigest_IsDeterministic()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest { ArtifactId = "pkg:maven/test@1.0" };
+
+        // Act
+        var result1 = facade.ComputeTrustScore(request);
+        var result2 = facade.ComputeTrustScore(request);
+
+        // Assert
+        result1.Predicate.PolicyDigest.Should().Be(result2.Predicate.PolicyDigest);
+    }
+
+    #endregion
+
+    #region Basis Point Arithmetic Tests
+
+    [Fact]
+    public void ComputeTrustScore_WeightsSumTo10000()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest { ArtifactId = "pkg:maven/test@1.0" };
+
+        // Act
+        var result = facade.ComputeTrustScore(request);
+
+        // Assert
+        var weights = result.Predicate.WeightsUsed;
+        var sum = weights.BaseSeverity + weights.Reachability + weights.Evidence + weights.Provenance;
+        sum.Should().Be(10000, "weights must sum to 10000 basis points");
+    }
+
+    [Fact]
+    public void ComputeTrustScore_FinalScoreWithinBounds()
+    {
+        // Arrange
+        var facade = CreateFacade();
+
+        // Test multiple scenarios
+        var scenarios = new[]
+        {
+            new TrustScoreRequest { ArtifactId = "pkg:a@1.0" },
+            new TrustScoreRequest { ArtifactId = "pkg:b@1.0", VulnerabilityId = "CVE-2024-1234" },
+        };
+
+        foreach (var request in scenarios)
+        {
+            // Act
+            var result = facade.ComputeTrustScore(request);
+
+            // Assert
+            result.Predicate.TrustScoreBps.Should().BeInRange(0, 10000);
+        }
+    }
+
+    #endregion
+
+    #region Risk Tier Tests
+
+    [Theory]
+    [InlineData(9500, "Critical")]
+    [InlineData(8000, "High")]
+    [InlineData(5000, "Medium")]
+    [InlineData(2000, "Low")]
+    [InlineData(500, "Info")]
+    public void RiskTier_MapsCorrectly(int scoreBps, string expectedTier)
+    {
+        // The tier is derived from the score; verify tier naming
+        var tier = scoreBps switch
+        {
+            >= 9000 => RiskTier.Critical,
+            >= 7000 => RiskTier.High,
+            >= 4000 => RiskTier.Medium,
+            >= 1000 => RiskTier.Low,
+            _ => RiskTier.Info
+        };
+
+        tier.ToString().Should().Be(expectedTier);
+    }
+
+    #endregion
+
+    #region Determinism Tests
+
+    [Fact]
+    public void ComputeTrustScore_SameInputs_ProducesSameOutputs()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var signals = SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow())
+            with
+            {
+                Epss = SignalState<EpssEvidence>.Present(new EpssEvidence(0.35, 0.65)),
+                Reachability = SignalState<ReachabilityEvidence>.Present(
+                    new ReachabilityEvidence(ReachabilityStatus.Reachable, 2, 5, null))
+            };
+
+        var request = new TrustScoreRequest
+        {
+            ArtifactId = "pkg:maven/test@1.0",
+            VulnerabilityId = "CVE-2024-1234",
+            Signals = signals
+        };
+
+        // Act
+        var result1 = facade.ComputeTrustScore(request);
+        var result2 = facade.ComputeTrustScore(request);
+
+        // Assert
+        result1.Predicate.TrustScoreBps.Should().Be(result2.Predicate.TrustScoreBps);
+        result1.Predicate.LatticeVerdict.Should().Be(result2.Predicate.LatticeVerdict);
+        result1.Predicate.Dimensions.Should().BeEquivalentTo(result2.Predicate.Dimensions);
+        result1.Predicate.PolicyDigest.Should().Be(result2.Predicate.PolicyDigest);
+    }
+
+    #endregion
+
+    #region Async API Tests
+
+    [Fact]
+    public async Task ComputeTrustScoreAsync_ReturnsResult()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest { ArtifactId = "pkg:maven/test@1.0" };
+
+        // Act
+        var result = await facade.ComputeTrustScoreAsync(request);
+
+        // Assert
+        result.Success.Should().BeTrue();
+        result.Predicate.Should().NotBeNull();
+    }
+
+    #endregion
+
+    #region Error Handling Tests
+
+    [Fact]
+    public void ComputeTrustScore_NullArtifactId_Throws()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest { ArtifactId = null! };
+
+        // Act & Assert
+        Assert.Throws<ArgumentException>(() => facade.ComputeTrustScore(request));
+    }
+
+    [Fact]
+    public void ComputeTrustScore_EmptyArtifactId_Throws()
+    {
+        // Arrange
+        var facade = CreateFacade();
+        var request = new TrustScoreRequest { ArtifactId = "" };
+
+        // Act & Assert
+        Assert.Throws<ArgumentException>(() => facade.ComputeTrustScore(request));
+    }
+
+    #endregion
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/WeightManifest/WeightManifestCommandsTests.cs

@@ -0,0 +1,234 @@
+// -----------------------------------------------------------------------------
+// WeightManifestCommandsTests.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Unit tests for CLI weight commands
+// Description: Tests for list, validate, diff, activate, and hash commands.
+//              Uses temp directories for offline, deterministic execution.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring.WeightManifest;
+
+public sealed class WeightManifestCommandsTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly WeightManifestLoader _loader;
+    private readonly WeightManifestCommands _commands;
+
+    public WeightManifestCommandsTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"stella-wm-cmd-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_tempDir);
+
+        var options = Options.Create(new WeightManifestLoaderOptions
+        {
+            ManifestDirectory = _tempDir,
+            RequireComputedHash = false,
+            StrictHashVerification = false
+        });
+
+        _loader = new WeightManifestLoader(options, NullLogger<WeightManifestLoader>.Instance);
+        _commands = new WeightManifestCommands(_loader);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+        {
+            Directory.Delete(_tempDir, recursive: true);
+        }
+    }
+
+    private string WriteManifest(string filename, string version, string effectiveFrom,
+        double rch = 0.50, double mit = 0.50)
+    {
+        var path = Path.Combine(_tempDir, filename);
+        var json = $$"""
+            {
+              "schemaVersion": "1.0.0",
+              "version": "{{version}}",
+              "effectiveFrom": "{{effectiveFrom}}",
+              "profile": "production",
+              "contentHash": "sha256:auto",
+              "weights": {
+                "legacy": { "rch": {{rch}}, "mit": {{mit}} },
+                "advisory": {}
+              }
+            }
+            """;
+        File.WriteAllText(path, json);
+        return path;
+    }
+
+    // ── ListAsync ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ListAsync_EmptyDirectory_ReturnsEmptyEntries()
+    {
+        var result = await _commands.ListAsync();
+        Assert.True(result.Entries.IsEmpty);
+    }
+
+    [Fact]
+    public async Task ListAsync_ReturnsAllManifests()
+    {
+        WriteManifest("a.weights.json", "v1", "2026-01-01T00:00:00Z");
+        WriteManifest("b.weights.json", "v2", "2026-02-01T00:00:00Z");
+
+        var result = await _commands.ListAsync();
+
+        Assert.Equal(2, result.Entries.Length);
+    }
+
+    [Fact]
+    public async Task ListAsync_ReportsHashStatus()
+    {
+        WriteManifest("auto.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result = await _commands.ListAsync();
+
+        Assert.Equal("auto", result.Entries[0].HashStatus);
+    }
+
+    // ── ValidateAsync ────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ValidateAsync_AllManifests_AllValid()
+    {
+        WriteManifest("a.weights.json", "v1", "2026-01-01T00:00:00Z");
+        WriteManifest("b.weights.json", "v2", "2026-02-01T00:00:00Z");
+
+        var result = await _commands.ValidateAsync();
+
+        Assert.True(result.AllValid);
+        Assert.Equal(2, result.Entries.Length);
+    }
+
+    [Fact]
+    public async Task ValidateAsync_SpecificFile_ValidatesOnly()
+    {
+        var path = WriteManifest("a.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result = await _commands.ValidateAsync(path);
+
+        Assert.Single(result.Entries);
+        Assert.True(result.Entries[0].IsValid);
+    }
+
+    [Fact]
+    public async Task ValidateAsync_InvalidManifest_ReportsIssues()
+    {
+        var path = WriteManifest("bad.weights.json", "v1", "2026-01-01T00:00:00Z",
+            rch: 0.90, mit: 0.90); // Sum > 1.0
+
+        var result = await _commands.ValidateAsync(path);
+
+        Assert.False(result.AllValid);
+        Assert.False(result.Entries[0].IsValid);
+        Assert.Contains(result.Entries[0].Issues, i => i.Contains("Legacy weights sum"));
+    }
+
+    // ── DiffAsync ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DiffAsync_TwoFiles_ReturnsDiff()
+    {
+        var path1 = WriteManifest("a.weights.json", "v1", "2026-01-01T00:00:00Z", rch: 0.30, mit: 0.70);
+        var path2 = WriteManifest("b.weights.json", "v2", "2026-02-01T00:00:00Z", rch: 0.50, mit: 0.50);
+
+        var diff = await _commands.DiffAsync(path1, path2);
+
+        Assert.True(diff.HasDifferences);
+        Assert.Equal("v1", diff.FromVersion);
+        Assert.Equal("v2", diff.ToVersion);
+        Assert.Contains(diff.Differences, d => d.Path == "weights.legacy.rch");
+    }
+
+    [Fact]
+    public async Task DiffByVersionAsync_FindsByVersionString()
+    {
+        WriteManifest("a.weights.json", "v2026-01-01", "2026-01-01T00:00:00Z", rch: 0.30, mit: 0.70);
+        WriteManifest("b.weights.json", "v2026-02-01", "2026-02-01T00:00:00Z", rch: 0.50, mit: 0.50);
+
+        var diff = await _commands.DiffByVersionAsync("v2026-01-01", "v2026-02-01");
+
+        Assert.True(diff.HasDifferences);
+    }
+
+    [Fact]
+    public async Task DiffByVersionAsync_MissingVersion_Throws()
+    {
+        WriteManifest("a.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        await Assert.ThrowsAsync<WeightManifestLoadException>(() =>
+            _commands.DiffByVersionAsync("v1", "v-nonexistent"));
+    }
+
+    // ── ActivateAsync ────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ActivateAsync_NoManifests_ReturnsNotFound()
+    {
+        var result = await _commands.ActivateAsync(DateTimeOffset.UtcNow);
+
+        Assert.False(result.Found);
+        Assert.Null(result.Version);
+    }
+
+    [Fact]
+    public async Task ActivateAsync_SelectsCorrectManifest()
+    {
+        WriteManifest("a.weights.json", "v2026-01-01", "2026-01-01T00:00:00Z");
+        WriteManifest("b.weights.json", "v2026-02-01", "2026-02-01T00:00:00Z");
+
+        var result = await _commands.ActivateAsync(DateTimeOffset.Parse("2026-01-15T00:00:00Z"));
+
+        Assert.True(result.Found);
+        Assert.Equal("v2026-01-01", result.Version);
+    }
+
+    // ── HashAsync ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task HashAsync_ComputesHash()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result = await _commands.HashAsync(path);
+
+        Assert.StartsWith("sha256:", result.ComputedHash);
+        Assert.True(result.HadPlaceholder);
+        Assert.False(result.WrittenBack);
+    }
+
+    [Fact]
+    public async Task HashAsync_WriteBack_ReplacesPlaceholder()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result = await _commands.HashAsync(path, writeBack: true);
+
+        Assert.True(result.WrittenBack);
+
+        var updatedContent = File.ReadAllText(path);
+        Assert.DoesNotContain("sha256:auto", updatedContent);
+        Assert.Contains(result.ComputedHash, updatedContent);
+    }
+
+    [Fact]
+    public async Task HashAsync_WriteBack_Idempotent()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result1 = await _commands.HashAsync(path, writeBack: true);
+        var result2 = await _commands.HashAsync(path, writeBack: false);
+
+        // Hash computed from already-replaced content should be the same
+        Assert.Equal(result1.ComputedHash, result2.ComputedHash);
+        Assert.False(result2.HadPlaceholder); // Placeholder is gone now
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/WeightManifest/WeightManifestHashComputerTests.cs

@@ -0,0 +1,215 @@
+// -----------------------------------------------------------------------------
+// WeightManifestHashComputerTests.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Unit tests for content hash computation
+// Description: Deterministic tests for SHA-256 content hashing of weight
+//              manifests, including canonical serialization and auto-replace.
+// -----------------------------------------------------------------------------
+
+using StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring.WeightManifest;
+
+public sealed class WeightManifestHashComputerTests
+{
+    private const string MinimalManifest = """
+        {
+          "schemaVersion": "1.0.0",
+          "version": "v2026-01-01",
+          "effectiveFrom": "2026-01-01T00:00:00Z",
+          "contentHash": "sha256:auto",
+          "weights": {
+            "legacy": { "rch": 0.50, "mit": 0.50 },
+            "advisory": {}
+          }
+        }
+        """;
+
+    // ── Determinism ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void ComputeFromJson_IsDeterministic_SameInputSameHash()
+    {
+        var hash1 = WeightManifestHashComputer.ComputeFromJson(MinimalManifest);
+        var hash2 = WeightManifestHashComputer.ComputeFromJson(MinimalManifest);
+
+        Assert.Equal(hash1, hash2);
+    }
+
+    [Fact]
+    public void ComputeFromJson_ReturnsCorrectFormat()
+    {
+        var hash = WeightManifestHashComputer.ComputeFromJson(MinimalManifest);
+
+        Assert.StartsWith("sha256:", hash);
+        Assert.Equal(71, hash.Length); // "sha256:" (7) + 64 hex chars
+    }
+
+    [Fact]
+    public void ComputeFromJson_ExcludesContentHashField()
+    {
+        // Two manifests identical except for contentHash should produce same hash
+        var manifestWithAuto = """
+            {
+              "schemaVersion": "1.0.0",
+              "version": "v2026-01-01",
+              "contentHash": "sha256:auto",
+              "weights": { "legacy": {}, "advisory": {} }
+            }
+            """;
+
+        var manifestWithDifferentHash = """
+            {
+              "schemaVersion": "1.0.0",
+              "version": "v2026-01-01",
+              "contentHash": "sha256:aaaa",
+              "weights": { "legacy": {}, "advisory": {} }
+            }
+            """;
+
+        var hash1 = WeightManifestHashComputer.ComputeFromJson(manifestWithAuto);
+        var hash2 = WeightManifestHashComputer.ComputeFromJson(manifestWithDifferentHash);
+
+        Assert.Equal(hash1, hash2);
+    }
+
+    [Fact]
+    public void ComputeFromJson_DifferentContent_DifferentHash()
+    {
+        var manifestA = """
+            { "schemaVersion": "1.0.0", "version": "v1", "contentHash": "sha256:auto",
+              "weights": { "legacy": { "rch": 0.30 }, "advisory": {} } }
+            """;
+
+        var manifestB = """
+            { "schemaVersion": "1.0.0", "version": "v1", "contentHash": "sha256:auto",
+              "weights": { "legacy": { "rch": 0.70 }, "advisory": {} } }
+            """;
+
+        var hashA = WeightManifestHashComputer.ComputeFromJson(manifestA);
+        var hashB = WeightManifestHashComputer.ComputeFromJson(manifestB);
+
+        Assert.NotEqual(hashA, hashB);
+    }
+
+    [Fact]
+    public void ComputeFromJson_PropertyOrderDoesNotMatter()
+    {
+        // JSON with properties in different order should produce same hash
+        var manifestOrdered = """
+            {
+              "schemaVersion": "1.0.0",
+              "version": "v1",
+              "contentHash": "sha256:auto",
+              "weights": { "legacy": { "a": 0.5, "b": 0.5 }, "advisory": {} }
+            }
+            """;
+
+        var manifestReversed = """
+            {
+              "weights": { "advisory": {}, "legacy": { "b": 0.5, "a": 0.5 } },
+              "contentHash": "sha256:auto",
+              "version": "v1",
+              "schemaVersion": "1.0.0"
+            }
+            """;
+
+        var hash1 = WeightManifestHashComputer.ComputeFromJson(manifestOrdered);
+        var hash2 = WeightManifestHashComputer.ComputeFromJson(manifestReversed);
+
+        Assert.Equal(hash1, hash2);
+    }
+
+    // ── Verify ───────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Verify_AutoPlaceholder_ReturnsFalse()
+    {
+        Assert.False(WeightManifestHashComputer.Verify(MinimalManifest, "sha256:auto"));
+    }
+
+    [Fact]
+    public void Verify_EmptyHash_ReturnsFalse()
+    {
+        Assert.False(WeightManifestHashComputer.Verify(MinimalManifest, ""));
+    }
+
+    [Fact]
+    public void Verify_CorrectHash_ReturnsTrue()
+    {
+        var hash = WeightManifestHashComputer.ComputeFromJson(MinimalManifest);
+        Assert.True(WeightManifestHashComputer.Verify(MinimalManifest, hash));
+    }
+
+    [Fact]
+    public void Verify_WrongHash_ReturnsFalse()
+    {
+        Assert.False(WeightManifestHashComputer.Verify(
+            MinimalManifest, "sha256:0000000000000000000000000000000000000000000000000000000000000000"));
+    }
+
+    // ── ReplaceAutoHash ──────────────────────────────────────────────────
+
+    [Fact]
+    public void ReplaceAutoHash_ReplacesPlaceholder()
+    {
+        var (updatedJson, computedHash) = WeightManifestHashComputer.ReplaceAutoHash(MinimalManifest);
+
+        Assert.DoesNotContain("sha256:auto", updatedJson);
+        Assert.Contains(computedHash, updatedJson);
+    }
+
+    [Fact]
+    public void ReplaceAutoHash_ComputedHashVerifies()
+    {
+        var (updatedJson, computedHash) = WeightManifestHashComputer.ReplaceAutoHash(MinimalManifest);
+
+        // After replacement, the hash stored in the JSON should match
+        // what ComputeFromJson produces for the original content
+        Assert.True(WeightManifestHashComputer.Verify(updatedJson, computedHash));
+    }
+
+    // ── ComputeFromManifest ──────────────────────────────────────────────
+
+    [Fact]
+    public void ComputeFromManifest_ProducesSameHashAsComputeFromJson()
+    {
+        // Build a manifest that matches our minimal JSON
+        var manifest = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v2026-01-01",
+            EffectiveFrom = DateTimeOffset.Parse("2026-01-01T00:00:00Z"),
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights()
+        };
+
+        // Both paths should produce a valid sha256 hash
+        var hashFromManifest = WeightManifestHashComputer.ComputeFromManifest(manifest);
+        Assert.StartsWith("sha256:", hashFromManifest);
+        Assert.Equal(71, hashFromManifest.Length);
+    }
+
+    // ── Edge cases ───────────────────────────────────────────────────────
+
+    [Fact]
+    public void ComputeFromJson_ThrowsOnEmpty()
+    {
+        Assert.Throws<ArgumentException>(() =>
+            WeightManifestHashComputer.ComputeFromJson(""));
+    }
+
+    [Fact]
+    public void ComputeFromJson_ThrowsOnNull()
+    {
+        Assert.Throws<ArgumentException>(() =>
+            WeightManifestHashComputer.ComputeFromJson(null!));
+    }
+
+    [Fact]
+    public void ComputeFromManifest_ThrowsOnNull()
+    {
+        Assert.Throws<ArgumentNullException>(() =>
+            WeightManifestHashComputer.ComputeFromManifest(null!));
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Scoring/WeightManifest/WeightManifestLoaderTests.cs

@@ -0,0 +1,448 @@
+// -----------------------------------------------------------------------------
+// WeightManifestLoaderTests.cs
+// Sprint: SPRINT_20260208_051_Policy_versioned_weight_manifests
+// Task: T1 - Unit tests for manifest loader
+// Description: Tests for file-based manifest discovery, loading, validation,
+//              effectiveFrom selection, and diff computation. Uses temp dirs
+//              for offline, deterministic test execution.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using StellaOps.Policy.Determinization.Scoring.WeightManifest;
+
+namespace StellaOps.Policy.Determinization.Tests.Scoring.WeightManifest;
+
+public sealed class WeightManifestLoaderTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly WeightManifestLoader _loader;
+
+    public WeightManifestLoaderTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"stella-wm-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_tempDir);
+
+        var options = Options.Create(new WeightManifestLoaderOptions
+        {
+            ManifestDirectory = _tempDir,
+            RequireComputedHash = false,
+            StrictHashVerification = false
+        });
+
+        _loader = new WeightManifestLoader(options, NullLogger<WeightManifestLoader>.Instance);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+        {
+            Directory.Delete(_tempDir, recursive: true);
+        }
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────────────
+
+    private string WriteManifest(string filename, string version, string effectiveFrom,
+        double rch = 0.50, double mit = 0.50, string contentHash = "sha256:auto")
+    {
+        var path = Path.Combine(_tempDir, filename);
+        var json = $$"""
+            {
+              "schemaVersion": "1.0.0",
+              "version": "{{version}}",
+              "effectiveFrom": "{{effectiveFrom}}",
+              "profile": "production",
+              "contentHash": "{{contentHash}}",
+              "weights": {
+                "legacy": { "rch": {{rch}}, "mit": {{mit}} },
+                "advisory": {}
+              }
+            }
+            """;
+        File.WriteAllText(path, json);
+        return path;
+    }
+
+    // ── ListAsync ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ListAsync_EmptyDirectory_ReturnsEmpty()
+    {
+        var results = await _loader.ListAsync();
+        Assert.True(results.IsEmpty);
+    }
+
+    [Fact]
+    public async Task ListAsync_DiscoversSingleManifest()
+    {
+        WriteManifest("v2026-01-01.weights.json", "v2026-01-01", "2026-01-01T00:00:00Z");
+
+        var results = await _loader.ListAsync();
+
+        Assert.Single(results);
+        Assert.Equal("v2026-01-01", results[0].Manifest.Version);
+    }
+
+    [Fact]
+    public async Task ListAsync_MultipleManifests_SortedByEffectiveFromDescending()
+    {
+        WriteManifest("v2026-01-01.weights.json", "v2026-01-01", "2026-01-01T00:00:00Z");
+        WriteManifest("v2026-02-01.weights.json", "v2026-02-01", "2026-02-01T00:00:00Z");
+        WriteManifest("v2026-01-15.weights.json", "v2026-01-15", "2026-01-15T00:00:00Z");
+
+        var results = await _loader.ListAsync();
+
+        Assert.Equal(3, results.Length);
+        Assert.Equal("v2026-02-01", results[0].Manifest.Version);
+        Assert.Equal("v2026-01-15", results[1].Manifest.Version);
+        Assert.Equal("v2026-01-01", results[2].Manifest.Version);
+    }
+
+    [Fact]
+    public async Task ListAsync_SkipsInvalidFiles()
+    {
+        WriteManifest("valid.weights.json", "v1", "2026-01-01T00:00:00Z");
+        File.WriteAllText(Path.Combine(_tempDir, "invalid.weights.json"), "not valid json {{{");
+
+        var results = await _loader.ListAsync();
+
+        Assert.Single(results);
+        Assert.Equal("v1", results[0].Manifest.Version);
+    }
+
+    [Fact]
+    public async Task ListAsync_NonexistentDirectory_ReturnsEmpty()
+    {
+        var options = Options.Create(new WeightManifestLoaderOptions
+        {
+            ManifestDirectory = Path.Combine(_tempDir, "nonexistent")
+        });
+        var loader = new WeightManifestLoader(options, NullLogger<WeightManifestLoader>.Instance);
+
+        var results = await loader.ListAsync();
+
+        Assert.True(results.IsEmpty);
+    }
+
+    // ── LoadAsync ────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task LoadAsync_ValidFile_ReturnsLoadResult()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var result = await _loader.LoadAsync(path);
+
+        Assert.Equal("v1", result.Manifest.Version);
+        Assert.Equal("1.0.0", result.Manifest.SchemaVersion);
+        Assert.StartsWith("sha256:", result.ComputedHash);
+        Assert.False(result.HashVerified); // auto placeholder, not computed
+    }
+
+    [Fact]
+    public async Task LoadAsync_WithComputedHash_VerifiesCorrectly()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        // Load, compute hash, rewrite with correct hash
+        var json = File.ReadAllText(path);
+        var (updatedJson, computedHash) = WeightManifestHashComputer.ReplaceAutoHash(json);
+        File.WriteAllText(path, updatedJson);
+
+        var result = await _loader.LoadAsync(path);
+
+        Assert.True(result.HashVerified);
+        Assert.Equal(computedHash, result.ComputedHash);
+    }
+
+    [Fact]
+    public async Task LoadAsync_NonexistentFile_Throws()
+    {
+        await Assert.ThrowsAsync<WeightManifestLoadException>(() =>
+            _loader.LoadAsync(Path.Combine(_tempDir, "missing.json")));
+    }
+
+    [Fact]
+    public async Task LoadAsync_InvalidJson_Throws()
+    {
+        var path = Path.Combine(_tempDir, "bad.json");
+        File.WriteAllText(path, "not json");
+
+        await Assert.ThrowsAsync<WeightManifestLoadException>(() =>
+            _loader.LoadAsync(path));
+    }
+
+    [Fact]
+    public async Task LoadAsync_StrictMode_HashMismatch_Throws()
+    {
+        var path = WriteManifest("test.weights.json", "v1", "2026-01-01T00:00:00Z",
+            contentHash: "sha256:0000000000000000000000000000000000000000000000000000000000000000");
+
+        var strictOptions = Options.Create(new WeightManifestLoaderOptions
+        {
+            ManifestDirectory = _tempDir,
+            StrictHashVerification = true
+        });
+        var strictLoader = new WeightManifestLoader(strictOptions, NullLogger<WeightManifestLoader>.Instance);
+
+        await Assert.ThrowsAsync<WeightManifestLoadException>(() =>
+            strictLoader.LoadAsync(path));
+    }
+
+    // ── SelectEffectiveAsync ─────────────────────────────────────────────
+
+    [Fact]
+    public async Task SelectEffectiveAsync_NoManifests_ReturnsNull()
+    {
+        var result = await _loader.SelectEffectiveAsync(DateTimeOffset.UtcNow);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task SelectEffectiveAsync_SelectsMostRecentEffective()
+    {
+        WriteManifest("a.weights.json", "v2026-01-01", "2026-01-01T00:00:00Z");
+        WriteManifest("b.weights.json", "v2026-02-01", "2026-02-01T00:00:00Z");
+        WriteManifest("c.weights.json", "v2026-03-01", "2026-03-01T00:00:00Z");
+
+        var referenceDate = DateTimeOffset.Parse("2026-02-15T00:00:00Z");
+        var result = await _loader.SelectEffectiveAsync(referenceDate);
+
+        Assert.NotNull(result);
+        Assert.Equal("v2026-02-01", result.Manifest.Version);
+    }
+
+    [Fact]
+    public async Task SelectEffectiveAsync_DateBeforeAll_ReturnsNull()
+    {
+        WriteManifest("a.weights.json", "v2026-06-01", "2026-06-01T00:00:00Z");
+
+        var referenceDate = DateTimeOffset.Parse("2026-01-01T00:00:00Z");
+        var result = await _loader.SelectEffectiveAsync(referenceDate);
+
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task SelectEffectiveAsync_ExactDate_Matches()
+    {
+        WriteManifest("a.weights.json", "v2026-01-15", "2026-01-15T00:00:00Z");
+
+        var referenceDate = DateTimeOffset.Parse("2026-01-15T00:00:00Z");
+        var result = await _loader.SelectEffectiveAsync(referenceDate);
+
+        Assert.NotNull(result);
+        Assert.Equal("v2026-01-15", result.Manifest.Version);
+    }
+
+    // ── Validate ─────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Validate_ValidManifest_NoIssues()
+    {
+        var path = WriteManifest("valid.weights.json", "v1", "2026-01-01T00:00:00Z");
+        var result = await _loader.LoadAsync(path);
+
+        var issues = _loader.Validate(result);
+
+        Assert.True(issues.IsEmpty);
+    }
+
+    [Fact]
+    public async Task Validate_UnsupportedSchema_ReportsIssue()
+    {
+        var path = Path.Combine(_tempDir, "bad-schema.weights.json");
+        File.WriteAllText(path, """
+            {
+              "schemaVersion": "2.0.0",
+              "version": "v1",
+              "effectiveFrom": "2026-01-01T00:00:00Z",
+              "contentHash": "sha256:auto",
+              "weights": { "legacy": {}, "advisory": {} }
+            }
+            """);
+
+        var result = await _loader.LoadAsync(path);
+        var issues = _loader.Validate(result);
+
+        Assert.Single(issues);
+        Assert.Contains("Unsupported schema version", issues[0]);
+    }
+
+    [Fact]
+    public async Task Validate_UnnormalizedLegacyWeights_ReportsIssue()
+    {
+        var path = WriteManifest("bad-weights.weights.json", "v1", "2026-01-01T00:00:00Z",
+            rch: 0.80, mit: 0.80); // Sum = 1.60
+
+        var result = await _loader.LoadAsync(path);
+        var issues = _loader.Validate(result);
+
+        Assert.Contains(issues, i => i.Contains("Legacy weights sum"));
+    }
+
+    [Fact]
+    public async Task Validate_RequireComputedHash_AutoPlaceholder_ReportsIssue()
+    {
+        var path = WriteManifest("auto.weights.json", "v1", "2026-01-01T00:00:00Z");
+
+        var strictOptions = Options.Create(new WeightManifestLoaderOptions
+        {
+            ManifestDirectory = _tempDir,
+            RequireComputedHash = true
+        });
+        var strictLoader = new WeightManifestLoader(strictOptions, NullLogger<WeightManifestLoader>.Instance);
+
+        var result = await strictLoader.LoadAsync(path);
+        var issues = strictLoader.Validate(result);
+
+        Assert.Contains(issues, i => i.Contains("sha256:auto"));
+    }
+
+    // ── Diff ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Diff_IdenticalManifests_NoDifferences()
+    {
+        var manifest = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.Parse("2026-01-01T00:00:00Z"),
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights
+            {
+                Legacy = ImmutableDictionary<string, double>.Empty.Add("rch", 0.50).Add("mit", 0.50),
+                Advisory = ImmutableDictionary<string, double>.Empty
+            }
+        };
+
+        var diff = _loader.Diff(manifest, manifest);
+
+        Assert.False(diff.HasDifferences);
+    }
+
+    [Fact]
+    public void Diff_DifferentVersions_ShowsDifference()
+    {
+        var from = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.Parse("2026-01-01T00:00:00Z"),
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights()
+        };
+
+        var to = from with { Version = "v2" };
+
+        var diff = _loader.Diff(from, to);
+
+        Assert.True(diff.HasDifferences);
+        Assert.Contains(diff.Differences, d => d.Path == "version" && d.OldValue == "v1" && d.NewValue == "v2");
+    }
+
+    [Fact]
+    public void Diff_DifferentWeights_ShowsDifferences()
+    {
+        var from = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.Parse("2026-01-01T00:00:00Z"),
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights
+            {
+                Legacy = ImmutableDictionary<string, double>.Empty.Add("rch", 0.30),
+                Advisory = ImmutableDictionary<string, double>.Empty
+            }
+        };
+
+        var to = from with
+        {
+            Version = "v2",
+            Weights = new WeightManifestWeights
+            {
+                Legacy = ImmutableDictionary<string, double>.Empty.Add("rch", 0.50),
+                Advisory = ImmutableDictionary<string, double>.Empty
+            }
+        };
+
+        var diff = _loader.Diff(from, to);
+
+        Assert.True(diff.HasDifferences);
+        Assert.Contains(diff.Differences, d => d.Path == "weights.legacy.rch");
+        Assert.Equal("v1", diff.FromVersion);
+        Assert.Equal("v2", diff.ToVersion);
+    }
+
+    [Fact]
+    public void Diff_AddedWeight_ShowsAsNewField()
+    {
+        var from = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.Parse("2026-01-01T00:00:00Z"),
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights
+            {
+                Legacy = ImmutableDictionary<string, double>.Empty.Add("rch", 0.30),
+                Advisory = ImmutableDictionary<string, double>.Empty
+            }
+        };
+
+        var to = from with
+        {
+            Version = "v2",
+            Weights = new WeightManifestWeights
+            {
+                Legacy = ImmutableDictionary<string, double>.Empty
+                    .Add("rch", 0.30)
+                    .Add("mit", 0.20),
+                Advisory = ImmutableDictionary<string, double>.Empty
+            }
+        };
+
+        var diff = _loader.Diff(from, to);
+
+        Assert.True(diff.HasDifferences);
+        var mitDiff = diff.Differences.First(d => d.Path == "weights.legacy.mit");
+        Assert.Null(mitDiff.OldValue);
+        Assert.NotNull(mitDiff.NewValue);
+    }
+
+    // ── WeightManifestDocument model ─────────────────────────────────────
+
+    [Fact]
+    public void HasComputedHash_AutoPlaceholder_ReturnsFalse()
+    {
+        var manifest = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.UtcNow,
+            ContentHash = "sha256:auto",
+            Weights = new WeightManifestWeights()
+        };
+
+        Assert.False(manifest.HasComputedHash);
+    }
+
+    [Fact]
+    public void HasComputedHash_RealHash_ReturnsTrue()
+    {
+        var manifest = new WeightManifestDocument
+        {
+            SchemaVersion = "1.0.0",
+            Version = "v1",
+            EffectiveFrom = DateTimeOffset.UtcNow,
+            ContentHash = "sha256:abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789",
+            Weights = new WeightManifestWeights()
+        };
+
+        Assert.True(manifest.HasComputedHash);
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Engine.Tests/Integration/DeltaIfPresentIntegrationTests.cs

@@ -0,0 +1,281 @@
+// <copyright file="DeltaIfPresentIntegrationTests.cs" company="StellaOps">
+// SPDX-License-Identifier: BUSL-1.1
+// Sprint: SPRINT_20260208_043_Policy_delta_if_present_calculations_for_missing_signals (TSF-004)
+// Task: T2 - Wire API/CLI/UI integration tests
+// </copyright>
+
+using FluentAssertions;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Time.Testing;
+using StellaOps.Policy.Determinization;
+using StellaOps.Policy.Determinization.Evidence;
+using StellaOps.Policy.Determinization.Models;
+using StellaOps.Policy.Determinization.Scoring;
+using StellaOps.Policy.Engine.DependencyInjection;
+using Xunit;
+
+namespace StellaOps.Policy.Engine.Tests.Integration;
+
+/// <summary>
+/// Integration tests for delta-if-present service DI wiring and functionality.
+/// </summary>
+[Trait("Category", "Integration")]
+[Trait("Sprint", "20260208.043")]
+[Trait("Task", "T2")]
+public sealed class DeltaIfPresentIntegrationTests
+{
+    private readonly FakeTimeProvider _timeProvider = new();
+
+    private static ServiceCollection CreateServicesWithConfiguration()
+    {
+        var services = new ServiceCollection();
+        var configuration = new ConfigurationBuilder()
+            .AddInMemoryCollection()
+            .Build();
+        services.AddSingleton<IConfiguration>(configuration);
+        return services;
+    }
+
+    #region DI Wiring Tests
+
+    [Fact(DisplayName = "AddDeterminization registers IDeltaIfPresentCalculator")]
+    public void AddDeterminization_RegistersDeltaIfPresentCalculator()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+
+        // Act
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        // Assert
+        var calculator = provider.GetService<IDeltaIfPresentCalculator>();
+        calculator.Should().NotBeNull();
+    }
+
+    [Fact(DisplayName = "DeltaIfPresentCalculator is registered as singleton")]
+    public void DeltaIfPresentCalculator_IsRegisteredAsSingleton()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        // Act
+        var first = provider.GetService<IDeltaIfPresentCalculator>();
+        var second = provider.GetService<IDeltaIfPresentCalculator>();
+
+        // Assert
+        first.Should().BeSameAs(second);
+    }
+
+    [Fact(DisplayName = "AddDeterminizationEngine also registers delta-if-present")]
+    public void AddDeterminizationEngine_IncludesDeltaIfPresentCalculator()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminizationEngine();
+        var provider = services.BuildServiceProvider();
+
+        // Assert
+        var calculator = provider.GetService<IDeltaIfPresentCalculator>();
+        calculator.Should().NotBeNull();
+    }
+
+    #endregion
+
+    #region End-to-End Service Tests
+
+    [Fact(DisplayName = "CalculateSingleSignalDelta works through DI container")]
+    public void CalculateSingleSignalDelta_WorksThroughDI()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var result = calculator.CalculateSingleSignalDelta(snapshot, "VEX", 0.0);
+
+        // Assert
+        result.Should().NotBeNull();
+        result.Signal.Should().Be("VEX");
+        result.HypotheticalEntropy.Should().BeLessThan(result.CurrentEntropy);
+    }
+
+    [Fact(DisplayName = "CalculateFullAnalysis returns prioritized gaps")]
+    public void CalculateFullAnalysis_ReturnsPrioritizedGaps()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var analysis = calculator.CalculateFullAnalysis(snapshot);
+
+        // Assert
+        analysis.Should().NotBeNull();
+        analysis.GapAnalysis.Should().HaveCountGreaterThan(0);
+        analysis.PrioritizedGaps.Should().NotBeEmpty();
+    }
+
+    [Fact(DisplayName = "CalculateScoreBounds returns valid range")]
+    public void CalculateScoreBounds_ReturnsValidRange()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var bounds = calculator.CalculateScoreBounds(snapshot);
+
+        // Assert
+        bounds.Should().NotBeNull();
+        bounds.MinimumScore.Should().BeLessThanOrEqualTo(bounds.MaximumScore);
+        bounds.Range.Should().BeGreaterThan(0);
+    }
+
+    [Fact(DisplayName = "Calculator produces deterministic results through DI")]
+    public void Calculator_ProducesDeterministicResults()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var snapshot = CreatePartialSnapshot();
+
+        // Act
+        var result1 = calculator.CalculateSingleSignalDelta(snapshot, "EPSS", 0.5);
+        var result2 = calculator.CalculateSingleSignalDelta(snapshot, "EPSS", 0.5);
+
+        // Assert - Results should be identical
+        result1.CurrentScore.Should().Be(result2.CurrentScore);
+        result1.HypotheticalScore.Should().Be(result2.HypotheticalScore);
+        result1.CurrentEntropy.Should().Be(result2.CurrentEntropy);
+        result1.HypotheticalEntropy.Should().Be(result2.HypotheticalEntropy);
+    }
+
+    [Fact(DisplayName = "All signals can be analyzed without exceptions")]
+    public void AllSignals_CanBeAnalyzed()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var snapshot = CreateEmptySnapshot();
+        var signals = new[] { "VEX", "EPSS", "Reachability", "Runtime", "Backport", "SBOMLineage" };
+
+        // Act & Assert - All signals should be analyzable
+        foreach (var signal in signals)
+        {
+            var result = calculator.CalculateSingleSignalDelta(snapshot, signal, 0.5);
+            result.Signal.Should().Be(signal);
+            result.SignalWeight.Should().BeGreaterThan(0);
+        }
+    }
+
+    #endregion
+
+    #region Integration with Dependencies
+
+    [Fact(DisplayName = "Calculator uses injected UncertaintyScoreCalculator")]
+    public void Calculator_UsesInjectedDependencies()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        // Act - Get both services
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var uncertaintyCalc = provider.GetRequiredService<IUncertaintyScoreCalculator>();
+
+        // Assert - Both should be available
+        calculator.Should().NotBeNull();
+        uncertaintyCalc.Should().NotBeNull();
+    }
+
+    [Fact(DisplayName = "Calculator uses injected TrustScoreAggregator")]
+    public void Calculator_UsesInjectedTrustAggregator()
+    {
+        // Arrange
+        var services = CreateServicesWithConfiguration();
+        services.AddLogging();
+        services.AddSingleton<TimeProvider>(_timeProvider);
+        services.AddDeterminization();
+        var provider = services.BuildServiceProvider();
+
+        // Act - Get both services
+        var calculator = provider.GetRequiredService<IDeltaIfPresentCalculator>();
+        var aggregator = provider.GetRequiredService<TrustScoreAggregator>();
+
+        // Assert - Both should be available
+        calculator.Should().NotBeNull();
+        aggregator.Should().NotBeNull();
+    }
+
+    #endregion
+
+    #region Helpers
+
+    private SignalSnapshot CreateEmptySnapshot()
+    {
+        return SignalSnapshot.Empty("CVE-2024-1234", "pkg:maven/test@1.0", _timeProvider.GetUtcNow());
+    }
+
+    private SignalSnapshot CreatePartialSnapshot()
+    {
+        var now = _timeProvider.GetUtcNow();
+        return new SignalSnapshot
+        {
+            Cve = "CVE-2024-1234",
+            Purl = "pkg:maven/test@1.0",
+            Vex = SignalState<VexClaimSummary>.NotQueried(),
+            Epss = SignalState<EpssEvidence>.NotQueried(),
+            Reachability = SignalState<ReachabilityEvidence>.Queried(
+                new ReachabilityEvidence { Status = ReachabilityStatus.Reachable, AnalyzedAt = now }, now),
+            Runtime = SignalState<RuntimeEvidence>.NotQueried(),
+            Backport = SignalState<BackportEvidence>.NotQueried(),
+            Sbom = SignalState<SbomLineageEvidence>.Queried(
+                new SbomLineageEvidence { SbomDigest = "sha256:abc", Format = "SPDX", ComponentCount = 150, GeneratedAt = now, HasProvenance = true }, now),
+            Cvss = SignalState<CvssEvidence>.NotQueried(),
+            SnapshotAt = now
+        };
+    }
+
+    #endregion
+}

src/Policy/__Tests/StellaOps.Policy.Explainability.Tests/ProofGraphBuilderTests.cs

@@ -0,0 +1,509 @@
+// -----------------------------------------------------------------------------
+// ProofGraphBuilderTests.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T1 - Unit tests for proof graph builder
+// Description: Deterministic tests for proof graph construction, path finding,
+//              counterfactual overlays, and content-addressed IDs.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace StellaOps.Policy.Explainability.Tests;
+
+public sealed class ProofGraphBuilderTests
+{
+    private readonly ProofGraphBuilder _builder;
+
+    public ProofGraphBuilderTests()
+    {
+        _builder = new ProofGraphBuilder(NullLogger<ProofGraphBuilder>.Instance);
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────────────
+
+    private static VerdictRationale CreateTestRationale(
+        string cve = "CVE-2026-0001",
+        string verdict = "Affected",
+        double? score = 75.0,
+        bool includeReachability = true,
+        bool includeVex = true,
+        bool includeProvenance = true,
+        bool includePathWitness = false)
+    {
+        return new VerdictRationale
+        {
+            RationaleId = "rat:sha256:test",
+            VerdictRef = new VerdictReference
+            {
+                AttestationId = "att-001",
+                ArtifactDigest = "sha256:abc123",
+                PolicyId = "policy-001",
+                Cve = cve,
+                ComponentPurl = "pkg:npm/lodash@4.17.20"
+            },
+            Evidence = new RationaleEvidence
+            {
+                Cve = cve,
+                Component = new ComponentIdentity
+                {
+                    Purl = "pkg:npm/lodash@4.17.20",
+                    Name = "lodash",
+                    Version = "4.17.20",
+                    Ecosystem = "npm"
+                },
+                Reachability = includeReachability
+                    ? new ReachabilityDetail
+                    {
+                        VulnerableFunction = "merge()",
+                        EntryPoint = "app.js",
+                        PathSummary = "app.js -> utils.js -> lodash.merge()"
+                    }
+                    : null,
+                FormattedText = $"{cve} in lodash@4.17.20"
+            },
+            PolicyClause = new RationalePolicyClause
+            {
+                ClauseId = "S2.1",
+                RuleDescription = "Block on reachable critical CVEs",
+                Conditions = ["severity >= high", "reachability == direct"],
+                FormattedText = "Policy S2.1: Block on reachable critical CVEs"
+            },
+            Attestations = new RationaleAttestations
+            {
+                PathWitness = includePathWitness
+                    ? new AttestationReference
+                    {
+                        Id = "pw-001",
+                        Type = "path_witness",
+                        Digest = "sha256:pw1",
+                        Summary = "Path verified by static analysis"
+                    }
+                    : null,
+                VexStatements = includeVex
+                    ? [new AttestationReference
+                    {
+                        Id = "vex-001",
+                        Type = "vex",
+                        Digest = "sha256:vex1",
+                        Summary = "Vendor confirms affected"
+                    }]
+                    : null,
+                Provenance = includeProvenance
+                    ? new AttestationReference
+                    {
+                        Id = "prov-001",
+                        Type = "provenance",
+                        Digest = "sha256:prov1",
+                        Summary = "SLSA Level 3"
+                    }
+                    : null,
+                FormattedText = "Attestations verified"
+            },
+            Decision = new RationaleDecision
+            {
+                Verdict = verdict,
+                Score = score,
+                Recommendation = "Upgrade to lodash@4.17.21",
+                Mitigation = new MitigationGuidance
+                {
+                    Action = "upgrade",
+                    Details = "Patch available in 4.17.21"
+                },
+                FormattedText = $"{verdict} (score {score:F2})"
+            },
+            GeneratedAt = new DateTimeOffset(2026, 2, 9, 12, 0, 0, TimeSpan.Zero),
+            InputDigests = new RationaleInputDigests
+            {
+                VerdictDigest = "sha256:verdict1",
+                PolicyDigest = "sha256:policy1",
+                EvidenceDigest = "sha256:evidence1"
+            }
+        };
+    }
+
+    private static ScoreBreakdownDashboard CreateTestBreakdown()
+    {
+        return new ScoreBreakdownDashboard
+        {
+            DashboardId = "dash-001",
+            VerdictRef = new VerdictReference
+            {
+                AttestationId = "att-001",
+                ArtifactDigest = "sha256:abc123",
+                PolicyId = "policy-001"
+            },
+            CompositeScore = 75,
+            ActionBucket = "Schedule Next",
+            Factors =
+            [
+                new FactorContribution
+                {
+                    FactorId = "rch",
+                    FactorName = "Reachability",
+                    RawScore = 85,
+                    Weight = 0.30,
+                    Confidence = 0.90,
+                    Explanation = "Direct reachability confirmed"
+                },
+                new FactorContribution
+                {
+                    FactorId = "rts",
+                    FactorName = "Runtime Signal",
+                    RawScore = 60,
+                    Weight = 0.25,
+                    Confidence = 0.70,
+                    Explanation = "Runtime detection moderate"
+                },
+                new FactorContribution
+                {
+                    FactorId = "mit",
+                    FactorName = "Mitigation",
+                    RawScore = 30,
+                    Weight = 0.10,
+                    Confidence = 0.95,
+                    IsSubtractive = true,
+                    Explanation = "Patch available"
+                }
+            ],
+            GuardrailsApplied =
+            [
+                new GuardrailApplication
+                {
+                    GuardrailName = "speculativeCap",
+                    ScoreBefore = 80,
+                    ScoreAfter = 45,
+                    Reason = "No runtime evidence, capped at 45",
+                    Conditions = ["rch == 0", "rts == 0"]
+                }
+            ],
+            PreGuardrailScore = 80,
+            Entropy = 0.35,
+            NeedsReview = false,
+            ComputedAt = new DateTimeOffset(2026, 2, 9, 12, 0, 0, TimeSpan.Zero)
+        };
+    }
+
+    // ── Build basic graph ────────────────────────────────────────────────
+
+    [Fact]
+    public void Build_MinimalInput_CreatesGraph()
+    {
+        var rationale = CreateTestRationale(
+            includeReachability: false,
+            includeVex: false,
+            includeProvenance: false);
+
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Should().NotBeNull();
+        graph.GraphId.Should().StartWith("pg:sha256:");
+        graph.RootNodeId.Should().StartWith("verdict:");
+        graph.Nodes.Should().HaveCountGreaterThanOrEqualTo(2); // verdict + policy
+    }
+
+    [Fact]
+    public void Build_WithReachability_AddsEvidenceNode()
+    {
+        var rationale = CreateTestRationale(includeReachability: true);
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Nodes.Should().Contain(n => n.Type == ProofNodeType.ReachabilityAnalysis);
+        graph.LeafNodeIds.Should().Contain(id => id.Contains("reachability"));
+    }
+
+    [Fact]
+    public void Build_WithVex_AddsVexNode()
+    {
+        var rationale = CreateTestRationale(includeVex: true);
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Nodes.Should().Contain(n => n.Type == ProofNodeType.VexStatement);
+    }
+
+    [Fact]
+    public void Build_WithProvenance_AddsProvenanceNode()
+    {
+        var rationale = CreateTestRationale(includeProvenance: true);
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Nodes.Should().Contain(n => n.Type == ProofNodeType.Provenance);
+    }
+
+    [Fact]
+    public void Build_WithPathWitness_AddsPathWitnessNode()
+    {
+        var rationale = CreateTestRationale(includePathWitness: true);
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.LeafNodeIds.Should().Contain(id => id.Contains("pathwitness"));
+    }
+
+    // ── Score breakdown integration ──────────────────────────────────────
+
+    [Fact]
+    public void Build_WithScoreBreakdown_AddsScoreNodes()
+    {
+        var rationale = CreateTestRationale();
+        var breakdown = CreateTestBreakdown();
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ScoreBreakdown = breakdown,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Nodes.Should().Contain(n => n.Id == "score:rch");
+        graph.Nodes.Should().Contain(n => n.Id == "score:rts");
+        graph.Nodes.Should().Contain(n => n.Id == "score:mit");
+    }
+
+    [Fact]
+    public void Build_WithGuardrails_AddsGuardrailNodes()
+    {
+        var rationale = CreateTestRationale();
+        var breakdown = CreateTestBreakdown();
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ScoreBreakdown = breakdown,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.Nodes.Should().Contain(n => n.Type == ProofNodeType.Guardrail);
+        graph.Edges.Should().Contain(e => e.Relation == ProofEdgeRelation.GuardrailApplied);
+    }
+
+    // ── Determinism ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void Build_IsDeterministic_SameInputsSameGraphId()
+    {
+        var rationale = CreateTestRationale();
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph1 = _builder.Build(input);
+        var graph2 = _builder.Build(input);
+
+        graph1.GraphId.Should().Be(graph2.GraphId);
+        graph1.Nodes.Length.Should().Be(graph2.Nodes.Length);
+    }
+
+    [Fact]
+    public void Build_DifferentInputs_DifferentGraphIds()
+    {
+        var rationale1 = CreateTestRationale(cve: "CVE-2026-0001");
+        var rationale2 = CreateTestRationale(cve: "CVE-2026-0002");
+
+        var graph1 = _builder.Build(new ProofGraphInput
+        {
+            Rationale = rationale1,
+            ComputedAt = rationale1.GeneratedAt
+        });
+        var graph2 = _builder.Build(new ProofGraphInput
+        {
+            Rationale = rationale2,
+            ComputedAt = rationale2.GeneratedAt
+        });
+
+        graph1.GraphId.Should().NotBe(graph2.GraphId);
+    }
+
+    // ── Depth hierarchy ──────────────────────────────────────────────────
+
+    [Fact]
+    public void Build_NodeDepths_FollowHierarchy()
+    {
+        var rationale = CreateTestRationale();
+        var breakdown = CreateTestBreakdown();
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ScoreBreakdown = breakdown,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        var verdictNode = graph.Nodes.First(n => n.Type == ProofNodeType.Verdict);
+        var policyNode = graph.Nodes.First(n => n.Type == ProofNodeType.PolicyRule);
+        var scoreNodes = graph.Nodes.Where(n => n.Type == ProofNodeType.ScoreComputation);
+        var leafNodes = graph.Nodes.Where(n => n.Depth == 3);
+
+        verdictNode.Depth.Should().Be(0);
+        policyNode.Depth.Should().Be(1);
+        scoreNodes.Should().AllSatisfy(n => n.Depth.Should().Be(2));
+        leafNodes.Should().AllSatisfy(n => n.Depth.Should().Be(3));
+    }
+
+    // ── Critical paths ───────────────────────────────────────────────────
+
+    [Fact]
+    public void Build_FullEvidence_HasCriticalPaths()
+    {
+        var rationale = CreateTestRationale(
+            includeReachability: true,
+            includeVex: true,
+            includeProvenance: true);
+
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        graph.CriticalPaths.Should().NotBeEmpty();
+        graph.CriticalPaths.Should().Contain(p => p.IsCritical);
+    }
+
+    [Fact]
+    public void Build_CriticalPaths_StartFromLeafAndEndAtRoot()
+    {
+        var rationale = CreateTestRationale(includeVex: true);
+        var input = new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        };
+
+        var graph = _builder.Build(input);
+
+        foreach (var path in graph.CriticalPaths)
+        {
+            path.NodeIds.Should().NotBeEmpty();
+            graph.LeafNodeIds.Should().Contain(path.NodeIds[0]);
+            path.NodeIds[^1].Should().Be(graph.RootNodeId);
+        }
+    }
+
+    // ── Counterfactual overlay ───────────────────────────────────────────
+
+    [Fact]
+    public void AddCounterfactualOverlay_AddsCounterfactualNode()
+    {
+        var rationale = CreateTestRationale();
+        var breakdown = CreateTestBreakdown();
+        var baseGraph = _builder.Build(new ProofGraphInput
+        {
+            Rationale = rationale,
+            ScoreBreakdown = breakdown,
+            ComputedAt = rationale.GeneratedAt
+        });
+
+        var scenario = new CounterfactualScenario
+        {
+            Label = "Full Mitigation",
+            FactorOverrides = ImmutableDictionary<string, int>.Empty
+                .Add("mit", 100),
+            ResultingScore = 50
+        };
+
+        var overlayGraph = _builder.AddCounterfactualOverlay(baseGraph, scenario);
+
+        overlayGraph.Nodes.Should().Contain(n => n.Type == ProofNodeType.Counterfactual);
+        overlayGraph.GraphId.Should().NotBe(baseGraph.GraphId);
+    }
+
+    [Fact]
+    public void AddCounterfactualOverlay_ConnectsOverriddenFactors()
+    {
+        var rationale = CreateTestRationale();
+        var breakdown = CreateTestBreakdown();
+        var baseGraph = _builder.Build(new ProofGraphInput
+        {
+            Rationale = rationale,
+            ScoreBreakdown = breakdown,
+            ComputedAt = rationale.GeneratedAt
+        });
+
+        var scenario = new CounterfactualScenario
+        {
+            Label = "Patch Applied",
+            FactorOverrides = ImmutableDictionary<string, int>.Empty
+                .Add("mit", 100)
+                .Add("rch", 0),
+            ResultingScore = 30
+        };
+
+        var overlayGraph = _builder.AddCounterfactualOverlay(baseGraph, scenario);
+
+        overlayGraph.Edges.Should().Contain(e => e.Relation == ProofEdgeRelation.Overrides);
+    }
+
+    // ── Edge cases ───────────────────────────────────────────────────────
+
+    [Fact]
+    public void Build_ThrowsOnNullInput()
+    {
+        var act = () => _builder.Build(null!);
+        act.Should().Throw<ArgumentNullException>();
+    }
+
+    [Fact]
+    public void AddCounterfactualOverlay_ThrowsOnNullGraph()
+    {
+        var scenario = new CounterfactualScenario
+        {
+            Label = "test",
+            FactorOverrides = ImmutableDictionary<string, int>.Empty
+        };
+
+        var act = () => _builder.AddCounterfactualOverlay(null!, scenario);
+        act.Should().Throw<ArgumentNullException>();
+    }
+
+    [Fact]
+    public void AddCounterfactualOverlay_ThrowsOnNullScenario()
+    {
+        var rationale = CreateTestRationale();
+        var graph = _builder.Build(new ProofGraphInput
+        {
+            Rationale = rationale,
+            ComputedAt = rationale.GeneratedAt
+        });
+
+        var act = () => _builder.AddCounterfactualOverlay(graph, null!);
+        act.Should().Throw<ArgumentNullException>();
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Explainability.Tests/ProofStudioServiceTests.cs

@@ -0,0 +1,277 @@
+// -----------------------------------------------------------------------------
+// ProofStudioServiceTests.cs
+// Sprint: SPRINT_20260208_049_Policy_proof_studio_ux
+// Task: T2 - Integration tests for proof studio service
+// Description: Tests for the ProofStudioService integration layer that
+//              composes proof graphs and score breakdowns from policy
+//              engine data.
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+using FluentAssertions;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace StellaOps.Policy.Explainability.Tests;
+
+public sealed class ProofStudioServiceTests
+{
+    private readonly IProofStudioService _service;
+
+    public ProofStudioServiceTests()
+    {
+        var services = new ServiceCollection();
+        services.AddVerdictExplainability();
+        services.AddLogging();
+        services.AddMetrics();
+
+        var provider = services.BuildServiceProvider();
+        _service = provider.GetRequiredService<IProofStudioService>();
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────────────
+
+    private static VerdictRationale CreateTestRationale(string cve = "CVE-2026-0001")
+    {
+        return new VerdictRationale
+        {
+            RationaleId = "rat:sha256:test",
+            VerdictRef = new VerdictReference
+            {
+                AttestationId = "att-001",
+                ArtifactDigest = "sha256:abc123",
+                PolicyId = "policy-001",
+                Cve = cve
+            },
+            Evidence = new RationaleEvidence
+            {
+                Cve = cve,
+                Component = new ComponentIdentity
+                {
+                    Purl = "pkg:npm/lodash@4.17.20",
+                    Name = "lodash",
+                    Version = "4.17.20",
+                    Ecosystem = "npm"
+                },
+                Reachability = new ReachabilityDetail
+                {
+                    VulnerableFunction = "merge()",
+                    EntryPoint = "app.js",
+                    PathSummary = "app.js -> lodash.merge()"
+                },
+                FormattedText = $"{cve} in lodash@4.17.20"
+            },
+            PolicyClause = new RationalePolicyClause
+            {
+                ClauseId = "S2.1",
+                RuleDescription = "Block on reachable critical CVEs",
+                Conditions = ["severity >= high"],
+                FormattedText = "Policy S2.1"
+            },
+            Attestations = new RationaleAttestations
+            {
+                VexStatements =
+                [
+                    new AttestationReference
+                    {
+                        Id = "vex-001", Type = "vex",
+                        Digest = "sha256:vex1",
+                        Summary = "Vendor confirms affected"
+                    }
+                ],
+                FormattedText = "Attestations verified"
+            },
+            Decision = new RationaleDecision
+            {
+                Verdict = "Affected",
+                Score = 75.0,
+                Recommendation = "Upgrade lodash",
+                FormattedText = "Affected (score 75.00)"
+            },
+            GeneratedAt = new DateTimeOffset(2026, 2, 9, 12, 0, 0, TimeSpan.Zero),
+            InputDigests = new RationaleInputDigests
+            {
+                VerdictDigest = "sha256:verdict1",
+                PolicyDigest = "sha256:policy1"
+            }
+        };
+    }
+
+    private static ProofStudioRequest CreateFullRequest()
+    {
+        return new ProofStudioRequest
+        {
+            Rationale = CreateTestRationale(),
+            CompositeScore = 75,
+            ActionBucket = "Schedule Next",
+            ScoreFactors =
+            [
+                new ScoreFactorInput
+                {
+                    Factor = "reachability",
+                    Value = 85,
+                    Weight = 0.30,
+                    Confidence = 0.90,
+                    Reason = "Direct reachability confirmed"
+                },
+                new ScoreFactorInput
+                {
+                    Factor = "evidence",
+                    Value = 60,
+                    Weight = 0.25,
+                    Confidence = 0.70,
+                    Reason = "Runtime evidence moderate"
+                },
+                new ScoreFactorInput
+                {
+                    Factor = "mitigation",
+                    Value = 30,
+                    Weight = 0.10,
+                    Confidence = 0.95,
+                    IsSubtractive = true,
+                    Reason = "Patch available"
+                }
+            ],
+            Guardrails =
+            [
+                new GuardrailInput
+                {
+                    Name = "speculativeCap",
+                    ScoreBefore = 80,
+                    ScoreAfter = 45,
+                    Reason = "No runtime evidence, capped",
+                    Conditions = ["rch == 0"]
+                }
+            ],
+            Entropy = 0.35,
+            NeedsReview = false
+        };
+    }
+
+    // ── Compose tests ────────────────────────────────────────────────────
+
+    [Fact]
+    public void Compose_MinimalRequest_ReturnsView()
+    {
+        var request = new ProofStudioRequest
+        {
+            Rationale = CreateTestRationale()
+        };
+
+        var view = _service.Compose(request);
+
+        view.Should().NotBeNull();
+        view.ProofGraph.Should().NotBeNull();
+        view.ProofGraph.GraphId.Should().StartWith("pg:sha256:");
+        view.ScoreBreakdown.Should().BeNull();
+    }
+
+    [Fact]
+    public void Compose_WithScoreFactors_BuildsDashboard()
+    {
+        var request = CreateFullRequest();
+
+        var view = _service.Compose(request);
+
+        view.ScoreBreakdown.Should().NotBeNull();
+        view.ScoreBreakdown!.Factors.Should().HaveCount(3);
+        view.ScoreBreakdown.CompositeScore.Should().Be(75);
+        view.ScoreBreakdown.ActionBucket.Should().Be("Schedule Next");
+    }
+
+    [Fact]
+    public void Compose_WithGuardrails_IncludesGuardrailsInDashboard()
+    {
+        var request = CreateFullRequest();
+
+        var view = _service.Compose(request);
+
+        view.ScoreBreakdown!.GuardrailsApplied.Should().HaveCount(1);
+        view.ScoreBreakdown.GuardrailsApplied[0].GuardrailName.Should().Be("speculativeCap");
+    }
+
+    [Fact]
+    public void Compose_FactorNamesAreFormatted()
+    {
+        var request = CreateFullRequest();
+
+        var view = _service.Compose(request);
+
+        var names = view.ScoreBreakdown!.Factors.Select(f => f.FactorName).ToArray();
+        names.Should().Contain("Reachability");
+        names.Should().Contain("Evidence");
+        names.Should().Contain("Mitigation");
+    }
+
+    [Fact]
+    public void Compose_GraphContainsScoreNodes()
+    {
+        var request = CreateFullRequest();
+
+        var view = _service.Compose(request);
+
+        view.ProofGraph.Nodes.Should().Contain(n => n.Id == "score:reachability");
+        view.ProofGraph.Nodes.Should().Contain(n => n.Id == "score:evidence");
+        view.ProofGraph.Nodes.Should().Contain(n => n.Id == "score:mitigation");
+    }
+
+    [Fact]
+    public void Compose_ThrowsOnNullRequest()
+    {
+        var act = () => _service.Compose(null!);
+        act.Should().Throw<ArgumentNullException>();
+    }
+
+    // ── Counterfactual tests ─────────────────────────────────────────────
+
+    [Fact]
+    public void ApplyCounterfactual_AddsOverlay()
+    {
+        var request = CreateFullRequest();
+        var view = _service.Compose(request);
+
+        var scenario = new CounterfactualScenario
+        {
+            Label = "Full Patch",
+            FactorOverrides = ImmutableDictionary<string, int>.Empty
+                .Add("mitigation", 100),
+            ResultingScore = 50
+        };
+
+        var updatedView = _service.ApplyCounterfactual(view, scenario);
+
+        updatedView.ProofGraph.Nodes.Should()
+            .Contain(n => n.Type == ProofNodeType.Counterfactual);
+        updatedView.ProofGraph.GraphId.Should()
+            .NotBe(view.ProofGraph.GraphId);
+    }
+
+    [Fact]
+    public void ApplyCounterfactual_ThrowsOnNullView()
+    {
+        var scenario = new CounterfactualScenario
+        {
+            Label = "test",
+            FactorOverrides = ImmutableDictionary<string, int>.Empty
+        };
+
+        var act = () => _service.ApplyCounterfactual(null!, scenario);
+        act.Should().Throw<ArgumentNullException>();
+    }
+
+    // ── DI integration ───────────────────────────────────────────────────
+
+    [Fact]
+    public void DI_ResolvesAllExplainabilityServices()
+    {
+        var services = new ServiceCollection();
+        services.AddVerdictExplainability();
+        services.AddLogging();
+        services.AddMetrics();
+        var provider = services.BuildServiceProvider();
+
+        provider.GetService<IVerdictRationaleRenderer>().Should().NotBeNull();
+        provider.GetService<IProofGraphBuilder>().Should().NotBeNull();
+        provider.GetService<IProofStudioService>().Should().NotBeNull();
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Explainability.Tests/StellaOps.Policy.Explainability.Tests.csproj

@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" />
+    <PackageReference Include="FluentAssertions" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\__Libraries\StellaOps.Policy.Explainability\StellaOps.Policy.Explainability.csproj" />
+    <ProjectReference Include="..\..\..\__Libraries\StellaOps.TestKit\StellaOps.TestKit.csproj" />
+  </ItemGroup>
+
+</Project>