DET-001/002/003: Add IGuidProvider abstraction and refactor Policy.Unknowns for determinism

- Created IGuidProvider interface and SystemGuidProvider in StellaOps.Determinism.Abstractions
- Added SequentialGuidProvider for testing deterministic GUID generation
- Added DeterminismServiceCollectionExtensions with AddDeterminismDefaults()
- Refactored Policy.Unknowns:
  - UnknownsRepository now uses TimeProvider and IGuidProvider
  - BudgetExceededEventFactory accepts optional TimeProvider parameter
  - ServiceCollectionExtensions calls AddDeterminismDefaults()
- Fixed Policy.Exceptions csproj (added ImplicitUsings, Nullable, PackageReferences)

Sprint: SPRINT_20260104_001_BE_determinism_timeprovider_injection
Tasks: DET-001 (audit), DET-002 (IGuidProvider), DET-003 (registration pattern), DET-004 (partial - Policy.Unknowns)

src/Policy/__Libraries/StellaOps.Policy.Unknowns/Events/BudgetExceededEventFactory.cs

@@ -29,9 +29,15 @@ public static class BudgetExceededEventFactory
     /// <summary>
     /// Creates a budget exceeded notification event payload.
     /// </summary>
+    /// <param name="environment">The environment name.</param>
+    /// <param name="result">The budget check result.</param>
+    /// <param name="timeProvider">Time provider for timestamp. Uses system time if null.</param>
+    /// <param name="imageDigest">Optional image digest.</param>
+    /// <param name="policyRevisionId">Optional policy revision ID.</param>
     public static BudgetEventPayload CreatePayload(
         string environment,
         BudgetCheckResult result,
+        TimeProvider? timeProvider = null,
         string? imageDigest = null,
         string? policyRevisionId = null)
     {
@@ -57,7 +63,7 @@ public static class BudgetExceededEventFactory
             Message = result.Message,
             ImageDigest = imageDigest,
             PolicyRevisionId = policyRevisionId,
-            Timestamp = DateTimeOffset.UtcNow
+            Timestamp = (timeProvider ?? TimeProvider.System).GetUtcNow()
         };
     }
 
@@ -198,7 +204,7 @@ public sealed record BudgetEventPayload
     /// <summary>
     /// Event timestamp.
     /// </summary>
-    public DateTimeOffset Timestamp { get; init; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset Timestamp { get; init; }
 }
 
 /// <summary>