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src/Policy/StellaOps.Policy.Engine/Snapshots/SnapshotModels.cs

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+using System.Text.Json.Serialization;
+using StellaOps.Policy.Engine.Ledger;
+
+namespace StellaOps.Policy.Engine.Snapshots;
+
+internal sealed record SnapshotSummary(
+    [property: JsonPropertyName("snapshot_id")] string SnapshotId,
+    [property: JsonPropertyName("tenant_id")] string TenantId,
+    [property: JsonPropertyName("ledger_export_id")] string LedgerExportId,
+    [property: JsonPropertyName("generated_at")] string GeneratedAt,
+    [property: JsonPropertyName("status_counts")] IReadOnlyDictionary<string, int> StatusCounts);
+
+internal sealed record SnapshotDetail(
+    [property: JsonPropertyName("snapshot_id")] string SnapshotId,
+    [property: JsonPropertyName("tenant_id")] string TenantId,
+    [property: JsonPropertyName("ledger_export_id")] string LedgerExportId,
+    [property: JsonPropertyName("generated_at")] string GeneratedAt,
+    [property: JsonPropertyName("overlay_hash")] string OverlayHash,
+    [property: JsonPropertyName("status_counts")] IReadOnlyDictionary<string, int> StatusCounts,
+    [property: JsonPropertyName("records")] IReadOnlyList<LedgerExportRecord> Records);
+
+internal sealed record SnapshotRequest(
+    [property: JsonPropertyName("tenant_id")] string TenantId,
+    [property: JsonPropertyName("overlay_hash")] string OverlayHash);

src/Policy/StellaOps.Policy.Engine/Snapshots/SnapshotService.cs

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+using StellaOps.Policy.Engine.Ledger;
+using StellaOps.Policy.Engine.Orchestration;
+
+namespace StellaOps.Policy.Engine.Snapshots;
+
+/// <summary>
+/// Snapshot API stub (POLICY-ENGINE-35-201) built on ledger exports.
+/// </summary>
+internal sealed class SnapshotService
+{
+    private readonly TimeProvider _timeProvider;
+    private readonly LedgerExportService _ledger;
+    private readonly ISnapshotStore _store;
+
+    public SnapshotService(
+        TimeProvider timeProvider,
+        LedgerExportService ledger,
+        ISnapshotStore store)
+    {
+        _timeProvider = timeProvider ?? throw new ArgumentNullException(nameof(timeProvider));
+        _ledger = ledger ?? throw new ArgumentNullException(nameof(ledger));
+        _store = store ?? throw new ArgumentNullException(nameof(store));
+    }
+
+    public async Task<SnapshotDetail> CreateAsync(
+        SnapshotRequest request,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+
+        var exports = await _ledger.ListAsync(request.TenantId, cancellationToken).ConfigureAwait(false);
+        var export = exports.LastOrDefault();
+        if (export is null)
+        {
+            export = await _ledger.BuildAsync(new LedgerExportRequest(request.TenantId), cancellationToken)
+                .ConfigureAwait(false);
+        }
+
+        var statusCounts = export.Records
+            .GroupBy(r => r.Status)
+            .ToDictionary(g => g.Key, g => g.Count(), StringComparer.Ordinal);
+
+        var generatedAt = _timeProvider.GetUtcNow().ToString("O");
+        var snapshotId = StableIdGenerator.CreateUlid($"{export.Manifest.ExportId}|{request.OverlayHash}");
+
+        var snapshot = new SnapshotDetail(
+            SnapshotId: snapshotId,
+            TenantId: request.TenantId,
+            LedgerExportId: export.Manifest.ExportId,
+            GeneratedAt: generatedAt,
+            OverlayHash: request.OverlayHash,
+            StatusCounts: statusCounts,
+            Records: export.Records);
+
+        await _store.SaveAsync(snapshot, cancellationToken).ConfigureAwait(false);
+        return snapshot;
+    }
+
+    public Task<SnapshotDetail?> GetAsync(string snapshotId, CancellationToken cancellationToken = default)
+    {
+        return _store.GetAsync(snapshotId, cancellationToken);
+    }
+
+    public async Task<(IReadOnlyList<SnapshotSummary> Items, string? NextCursor)> ListAsync(
+        string? tenantId = null,
+        CancellationToken cancellationToken = default)
+    {
+        var snapshots = await _store.ListAsync(tenantId, cancellationToken).ConfigureAwait(false);
+        var summaries = snapshots
+            .OrderByDescending(s => s.GeneratedAt, StringComparer.Ordinal)
+            .Select(s => new SnapshotSummary(s.SnapshotId, s.TenantId, s.LedgerExportId, s.GeneratedAt, s.StatusCounts))
+            .ToList();
+
+        return (summaries, null);
+    }
+}

src/Policy/StellaOps.Policy.Engine/Snapshots/SnapshotStore.cs

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+using System.Collections.Concurrent;
+
+namespace StellaOps.Policy.Engine.Snapshots;
+
+internal interface ISnapshotStore
+{
+    Task SaveAsync(SnapshotDetail snapshot, CancellationToken cancellationToken = default);
+    Task<SnapshotDetail?> GetAsync(string snapshotId, CancellationToken cancellationToken = default);
+    Task<IReadOnlyList<SnapshotDetail>> ListAsync(string? tenantId = null, CancellationToken cancellationToken = default);
+}
+
+internal sealed class InMemorySnapshotStore : ISnapshotStore
+{
+    private readonly ConcurrentDictionary<string, SnapshotDetail> _snapshots = new(StringComparer.Ordinal);
+
+    public Task SaveAsync(SnapshotDetail snapshot, CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+        _snapshots[snapshot.SnapshotId] = snapshot;
+        return Task.CompletedTask;
+    }
+
+    public Task<SnapshotDetail?> GetAsync(string snapshotId, CancellationToken cancellationToken = default)
+    {
+        _snapshots.TryGetValue(snapshotId, out var value);
+        return Task.FromResult(value);
+    }
+
+    public Task<IReadOnlyList<SnapshotDetail>> ListAsync(string? tenantId = null, CancellationToken cancellationToken = default)
+    {
+        IEnumerable<SnapshotDetail> items = _snapshots.Values;
+        if (!string.IsNullOrWhiteSpace(tenantId))
+        {
+            items = items.Where(s => string.Equals(s.TenantId, tenantId, StringComparison.Ordinal));
+        }
+
+        var ordered = items
+            .OrderBy(s => s.GeneratedAt, StringComparer.Ordinal)
+            .ThenBy(s => s.SnapshotId, StringComparer.Ordinal)
+            .ToList();
+
+        return Task.FromResult<IReadOnlyList<SnapshotDetail>>(ordered);
+    }
+}