Complete batch 012 (golden set diff) and 013 (advisory chat), fix build errors

Sprints completed:
- SPRINT_20260110_012_* (golden set diff layer - 10 sprints)
- SPRINT_20260110_013_* (advisory chat - 4 sprints)

Build fixes applied:
- Fix namespace conflicts with Microsoft.Extensions.Options.Options.Create
- Fix VexDecisionReachabilityIntegrationTests API drift (major rewrite)
- Fix VexSchemaValidationTests FluentAssertions method name
- Fix FixChainGateIntegrationTests ambiguous type references
- Fix AdvisoryAI test files required properties and namespace aliases
- Add stub types for CveMappingController (ICveSymbolMappingService)
- Fix VerdictBuilderService static context issue

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Findings/StellaOps.Findings.Ledger.WebService/Program.cs

@@ -224,6 +224,10 @@ builder.Services.AddSingleton<IEvidenceContentService, NullEvidenceContentServic
 builder.Services.AddSingleton<IReachabilityMapService, NullReachabilityMapService>();
 builder.Services.AddSingleton<IRuntimeTimelineService, NullRuntimeTimelineService>();
 
+// Backport and runtime traces services (SPRINT_20260107_006_002_FE)
+builder.Services.AddSingleton<IRuntimeTracesService, NullRuntimeTracesService>();
+builder.Services.AddSingleton<IBackportEvidenceService, NullBackportEvidenceService>();
+
 // Alert and Decision services (SPRINT_3602)
 builder.Services.AddSingleton<IAlertService, AlertService>();
 builder.Services.AddSingleton<IDecisionService, DecisionService>();

src/Findings/StellaOps.Findings.Ledger.WebService/Services/NullBackportEvidenceService.cs

@@ -0,0 +1,28 @@
+// -----------------------------------------------------------------------------
+// NullBackportEvidenceService.cs
+// Sprint: SPRINT_20260107_006_002_FE_diff_runtime_tabs
+// Description: Stub implementation for backport evidence service
+// -----------------------------------------------------------------------------
+
+using StellaOps.Findings.Ledger.WebService.Contracts;
+using StellaOps.Findings.Ledger.WebService.Endpoints;
+
+namespace StellaOps.Findings.Ledger.WebService.Services;
+
+/// <summary>
+/// Stub implementation of IBackportEvidenceService that returns null (not found).
+/// </summary>
+public sealed class NullBackportEvidenceService : IBackportEvidenceService
+{
+    /// <inheritdoc/>
+    public Task<BackportEvidenceResponse?> GetBackportEvidenceAsync(Guid findingId, CancellationToken ct)
+    {
+        return Task.FromResult<BackportEvidenceResponse?>(null);
+    }
+
+    /// <inheritdoc/>
+    public Task<PatchesResponse?> GetPatchesAsync(Guid findingId, CancellationToken ct)
+    {
+        return Task.FromResult<PatchesResponse?>(null);
+    }
+}

src/Findings/StellaOps.Findings.Ledger.WebService/Services/NullRuntimeTracesService.cs

@@ -0,0 +1,31 @@
+// -----------------------------------------------------------------------------
+// NullRuntimeTracesService.cs
+// Sprint: SPRINT_20260107_006_002_FE_diff_runtime_tabs
+// Description: Stub implementation for runtime traces service
+// -----------------------------------------------------------------------------
+
+using StellaOps.Findings.Ledger.WebService.Contracts;
+using StellaOps.Findings.Ledger.WebService.Endpoints;
+
+namespace StellaOps.Findings.Ledger.WebService.Services;
+
+/// <summary>
+/// Stub implementation of IRuntimeTracesService that returns null (not found).
+/// </summary>
+public sealed class NullRuntimeTracesService : IRuntimeTracesService
+{
+    /// <inheritdoc/>
+    public Task<RuntimeTracesResponse?> GetTracesAsync(
+        Guid findingId,
+        RuntimeTracesQueryOptions options,
+        CancellationToken ct)
+    {
+        return Task.FromResult<RuntimeTracesResponse?>(null);
+    }
+
+    /// <inheritdoc/>
+    public Task<RtsScoreResponse?> GetRtsScoreAsync(Guid findingId, CancellationToken ct)
+    {
+        return Task.FromResult<RtsScoreResponse?>(null);
+    }
+}