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;
+    }
+}

src/__Libraries/StellaOps.Determinism.Abstractions/IGuidProvider.cs

@@ -0,0 +1,64 @@
+namespace StellaOps.Determinism;
+
+/// <summary>
+/// Abstraction for GUID generation to support deterministic testing.
+/// Inject this instead of using <see cref="Guid.NewGuid"/> directly.
+/// </summary>
+public interface IGuidProvider
+{
+    /// <summary>
+    /// Generates a new GUID.
+    /// </summary>
+    Guid NewGuid();
+}
+
+/// <summary>
+/// Default implementation using <see cref="Guid.NewGuid"/>.
+/// Register as singleton in DI: <c>services.AddSingleton&lt;IGuidProvider, SystemGuidProvider&gt;();</c>
+/// </summary>
+public sealed class SystemGuidProvider : IGuidProvider
+{
+    /// <summary>
+    /// Shared instance for non-DI scenarios.
+    /// </summary>
+    public static readonly SystemGuidProvider Instance = new();
+
+    /// <inheritdoc />
+    public Guid NewGuid() => Guid.NewGuid();
+}
+
+/// <summary>
+/// Deterministic GUID provider for testing. Returns GUIDs in a predictable sequence.
+/// </summary>
+public sealed class SequentialGuidProvider : IGuidProvider
+{
+    private int _counter;
+    private readonly Guid _baseGuid;
+
+    /// <summary>
+    /// Creates a sequential GUID provider starting from a base GUID.
+    /// </summary>
+    /// <param name="baseGuid">Optional base GUID. Defaults to all zeros.</param>
+    public SequentialGuidProvider(Guid? baseGuid = null)
+    {
+        _baseGuid = baseGuid ?? Guid.Empty;
+    }
+
+    /// <inheritdoc />
+    public Guid NewGuid()
+    {
+        var bytes = _baseGuid.ToByteArray();
+        var counter = Interlocked.Increment(ref _counter);
+        // Embed counter in last 4 bytes
+        bytes[12] = (byte)(counter >> 24);
+        bytes[13] = (byte)(counter >> 16);
+        bytes[14] = (byte)(counter >> 8);
+        bytes[15] = (byte)counter;
+        return new Guid(bytes);
+    }
+
+    /// <summary>
+    /// Resets the counter to zero.
+    /// </summary>
+    public void Reset() => Interlocked.Exchange(ref _counter, 0);
+}

src/__Libraries/StellaOps.Determinism.Abstractions/StellaOps.Determinism.Abstractions.csproj

@@ -9,4 +9,8 @@
     <Description>Attributes and abstractions for determinism enforcement in StellaOps.</Description>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
+  </ItemGroup>
 </Project>