Refactor SurfaceCacheValidator to simplify oldest entry calculation

Add global using for Xunit in test project

Enhance ImportValidatorTests with async validation and quarantine checks

Implement FileSystemQuarantineServiceTests for quarantine functionality

Add integration tests for ImportValidator to check monotonicity

Create BundleVersionTests to validate version parsing and comparison logic

Implement VersionMonotonicityCheckerTests for monotonicity checks and activation logic

src/Findings/StellaOps.Findings.Ledger/Services/IDecisionService.cs

@@ -0,0 +1,24 @@
+using StellaOps.Findings.Ledger.Domain;
+
+namespace StellaOps.Findings.Ledger.Services;
+
+/// <summary>
+/// Service for recording and querying triage decisions.
+/// </summary>
+public interface IDecisionService
+{
+    /// <summary>
+    /// Records a decision event (append-only, immutable).
+    /// </summary>
+    Task<DecisionEvent> RecordAsync(
+        DecisionEvent decision,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets decision history for an alert (immutable timeline).
+    /// </summary>
+    Task<IReadOnlyList<DecisionEvent>> GetHistoryAsync(
+        string tenantId,
+        string alertId,
+        CancellationToken cancellationToken = default);
+}