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src/Policy/__Tests/StellaOps.Policy.Engine.Tests/Fakes/FakeOverlayEventSink.cs

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+using StellaOps.Policy.Engine.Overlay;
+
+namespace StellaOps.Policy.Engine.Tests.Fakes;
+
+internal sealed class FakeOverlayEventSink : IOverlayEventSink
+{
+    public List<(string Topic, string Payload)> Published { get; } = new();
+
+    public Task PublishAsync(string topic, string payload, CancellationToken cancellationToken = default)
+    {
+        Published.Add((topic, payload));
+        return Task.CompletedTask;
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Engine.Tests/Fakes/FakeOverlayStore.cs

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+using StellaOps.Policy.Engine.Overlay;
+
+namespace StellaOps.Policy.Engine.Tests.Fakes;
+
+internal sealed class FakeOverlayStore : IOverlayStore
+{
+    public List<OverlayProjection> Saved { get; } = new();
+
+    public Task SaveAsync(OverlayProjection projection, CancellationToken cancellationToken = default)
+    {
+        Saved.Add(projection);
+        return Task.CompletedTask;
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Engine.Tests/OverlayProjectionServiceTests.cs

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+using System.Threading.Tasks;
+using StellaOps.Policy.Engine.Overlay;
+using StellaOps.Policy.Engine.Services;
+using StellaOps.Policy.Engine.Streaming;
+
+namespace StellaOps.Policy.Engine.Tests;
+
+public sealed class OverlayProjectionServiceTests
+{
+    [Fact]
+    public async Task BuildSnapshotAsync_ProducesHeaderAndSortedProjections()
+    {
+        var service = new OverlayProjectionService(new PolicyEvaluationService(), TimeProvider.System);
+
+        var request = new PathScopeSimulationRequest(
+            SchemaVersion: "1.0.0",
+            Tenant: "acme",
+            BasePolicyRef: "policy://acme/main@sha256:1",
+            CandidatePolicyRef: "policy://acme/feat@sha256:2",
+            Subject: new PathScopeSubject("pkg:npm/lodash@4.17.21", null, "lib.js", null),
+            Targets: new[]
+            {
+                new PathScopeTarget("b/file.js", "b/", "prefix", 0.5, null, null, null, "e2", null, null),
+                new PathScopeTarget("a/file.js", "a/", "prefix", 0.9, null, null, null, "e1", null, null)
+            },
+            Options: new SimulationOptions("path,finding,verdict", 100, IncludeTrace: true, Deterministic: true));
+
+        var lines = await service.BuildSnapshotAsync(request);
+
+        Assert.Equal(3, lines.Count); // header + 2 projections
+        Assert.Contains("overlay-projection-v1", lines[0]);
+        Assert.Contains("\"filePath\":\"a/file.js\"", lines[1]);
+        Assert.Contains("\"filePath\":\"b/file.js\"", lines[2]);
+    }
+}

src/Policy/__Tests/StellaOps.Policy.Engine.Tests/PathScopeSimulationBridgeServiceTests.cs

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+using System.Threading.Tasks;
+using StellaOps.Policy.Engine.Overlay;
+using StellaOps.Policy.Engine.Tests.Fakes;
+using StellaOps.Policy.Engine.Services;
+using StellaOps.Policy.Engine.Streaming;
+
+namespace StellaOps.Policy.Engine.Tests;
+
+public sealed class PathScopeSimulationBridgeServiceTests
+{
+    [Fact]
+    public async Task SimulateAsync_OrdersByInputAndProducesMetrics()
+    {
+        var bridge = CreateBridge();
+
+        var request = new PathScopeSimulationBridgeRequest(
+            Tenant: "acme",
+            Rules: Array.Empty<string>(),
+            Overlays: null,
+            Paths: new[]
+            {
+                BuildPathRequest("b/file.js"),
+                BuildPathRequest("a/file.js")
+            },
+            Mode: "preview",
+            Seed: 123);
+
+        var result = await bridge.SimulateAsync(request);
+
+        Assert.Equal(2, result.Decisions.Count);
+        Assert.Contains("\"filePath\":\"b/file.js\"", JsonSerializer.Serialize(result.Decisions[0].PathScope));
+        Assert.Contains("\"filePath\":\"a/file.js\"", JsonSerializer.Serialize(result.Decisions[1].PathScope));
+        Assert.Equal(2, result.Metrics.Evaluated);
+    }
+
+    [Fact]
+    public async Task SimulateAsync_WhatIfProducesDeltas()
+    {
+        var bridge = CreateBridge();
+
+        var request = new PathScopeSimulationBridgeRequest(
+            Tenant: "acme",
+            Rules: Array.Empty<string>(),
+            Overlays: null,
+            Paths: new[] { BuildPathRequest("a/file.js") },
+            Mode: "whatif",
+            Seed: 42);
+
+        var result = await bridge.SimulateAsync(request);
+
+        Assert.Single(result.Decisions);
+        Assert.Single(result.Deltas);
+    }
+
+    [Fact]
+    public async Task SimulateAsync_PublishesEventsAndSavesOverlays()
+    {
+        var sink = new FakeOverlayEventSink();
+        var store = new FakeOverlayStore();
+        var bridge = CreateBridge(sink, store);
+
+        var request = new PathScopeSimulationBridgeRequest(
+            Tenant: "acme",
+            Rules: Array.Empty<string>(),
+            Overlays: null,
+            Paths: new[] { BuildPathRequest("a/file.js") },
+            Mode: "whatif",
+            Seed: 99);
+
+        await bridge.SimulateAsync(request);
+
+        Assert.NotEmpty(sink.Published);
+        Assert.NotEmpty(store.Saved);
+    }
+
+    private static PathScopeSimulationBridgeService CreateBridge(
+        IOverlayEventSink? sink = null,
+        IOverlayStore? store = null)
+    {
+        sink ??= new FakeOverlayEventSink();
+        store ??= new FakeOverlayStore();
+
+        var overlayService = new OverlayProjectionService(new PolicyEvaluationService(), TimeProvider.System);
+        var publisher = new OverlayChangeEventPublisher(sink, TimeProvider.System);
+        return new PathScopeSimulationBridgeService(overlayService, publisher, store);
+    }
+
+    private static PathScopeSimulationRequest BuildPathRequest(string filePath) =>
+        new(
+            SchemaVersion: "1.0.0",
+            Tenant: "acme",
+            BasePolicyRef: "policy://acme/main@sha256:1",
+            CandidatePolicyRef: "policy://acme/feat@sha256:2",
+            Subject: new PathScopeSubject("pkg:npm/lodash@4.17.21", null, "lib.js", null),
+            Targets: new[]
+            {
+                new PathScopeTarget(filePath, "pat", "prefix", 0.9, null, null, null, "e1", null, null)
+            },
+            Options: new SimulationOptions("path,finding,verdict", 100, IncludeTrace: true, Deterministic: true));
+}