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/__Libraries/StellaOps.Determinism.Abstractions/DeterminismServiceCollectionExtensions.cs

@@ -0,0 +1,39 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+
+namespace StellaOps.Determinism;
+
+/// <summary>
+/// Extension methods for registering determinism abstractions in DI.
+/// </summary>
+public static class DeterminismServiceCollectionExtensions
+{
+    /// <summary>
+    /// Adds <see cref="TimeProvider.System"/> as a singleton.
+    /// </summary>
+    public static IServiceCollection AddSystemTimeProvider(this IServiceCollection services)
+    {
+        services.TryAddSingleton(TimeProvider.System);
+        return services;
+    }
+
+    /// <summary>
+    /// Adds <see cref="SystemGuidProvider"/> as the <see cref="IGuidProvider"/> singleton.
+    /// </summary>
+    public static IServiceCollection AddSystemGuidProvider(this IServiceCollection services)
+    {
+        services.TryAddSingleton<IGuidProvider, SystemGuidProvider>();
+        return services;
+    }
+
+    /// <summary>
+    /// Adds both <see cref="TimeProvider.System"/> and <see cref="SystemGuidProvider"/> as singletons.
+    /// This is the recommended setup for production services.
+    /// </summary>
+    public static IServiceCollection AddDeterminismDefaults(this IServiceCollection services)
+    {
+        services.AddSystemTimeProvider();
+        services.AddSystemGuidProvider();
+        return services;
+    }
+}