DET-004: Refactor Policy BudgetLedger for determinism

- Inject TimeProvider and IGuidProvider in BudgetLedger constructor
- Replace DateTimeOffset.UtcNow with _timeProvider.GetUtcNow()
- Replace Guid.NewGuid() with _guidProvider.NewGuid()
- Add Determinism.Abstractions reference to Policy csproj

Sprint: SPRINT_20260104_001_BE_determinism_timeprovider_injection
Task: DET-004 (in progress - Policy module)

src/Policy/__Libraries/StellaOps.Policy/Gates/BudgetLedger.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using StellaOps.Determinism;
 
 namespace StellaOps.Policy.Gates;
 
@@ -10,10 +11,18 @@ public sealed class BudgetLedger : IBudgetLedger
 {
     private readonly IBudgetStore _store;
     private readonly ILogger<BudgetLedger> _logger;
+    private readonly TimeProvider _timeProvider;
+    private readonly IGuidProvider _guidProvider;
 
-    public BudgetLedger(IBudgetStore store, ILogger<BudgetLedger>? logger = null)
+    public BudgetLedger(
+        IBudgetStore store,
+        TimeProvider? timeProvider = null,
+        IGuidProvider? guidProvider = null,
+        ILogger<BudgetLedger>? logger = null)
     {
         _store = store ?? throw new ArgumentNullException(nameof(store));
+        _timeProvider = timeProvider ?? TimeProvider.System;
+        _guidProvider = guidProvider ?? SystemGuidProvider.Instance;
         _logger = logger ?? NullLogger<BudgetLedger>.Instance;
     }
 
@@ -64,12 +73,12 @@ public sealed class BudgetLedger : IBudgetLedger
         // Record the consumption
         var entry = new BudgetEntry
         {
-            EntryId = Guid.NewGuid().ToString(),
+            EntryId = _guidProvider.NewGuid().ToString(),
             ServiceId = serviceId,
             Window = budget.Window,
             ReleaseId = releaseId,
             RiskPoints = riskPoints,
-            ConsumedAt = DateTimeOffset.UtcNow
+            ConsumedAt = _timeProvider.GetUtcNow()
         };
 
         await _store.AddEntryAsync(entry, ct).ConfigureAwait(false);
@@ -78,7 +87,7 @@ public sealed class BudgetLedger : IBudgetLedger
         var updatedBudget = budget with
         {
             Consumed = budget.Consumed + riskPoints,
-            UpdatedAt = DateTimeOffset.UtcNow
+            UpdatedAt = _timeProvider.GetUtcNow()
         };
 
         await _store.UpdateAsync(updatedBudget, ct).ConfigureAwait(false);
@@ -122,7 +131,7 @@ public sealed class BudgetLedger : IBudgetLedger
         var updatedBudget = budget with
         {
             Allocated = newAllocation,
-            UpdatedAt = DateTimeOffset.UtcNow
+            UpdatedAt = _timeProvider.GetUtcNow()
         };
 
         await _store.UpdateAsync(updatedBudget, ct).ConfigureAwait(false);
@@ -148,15 +157,15 @@ public sealed class BudgetLedger : IBudgetLedger
             Window = window,
             Allocated = DefaultBudgetAllocations.GetMonthlyAllocation(tier),
             Consumed = 0,
-            UpdatedAt = DateTimeOffset.UtcNow
+            UpdatedAt = _timeProvider.GetUtcNow()
         };
 
         await _store.CreateAsync(budget, ct).ConfigureAwait(false);
         return budget;
     }
 
-    private static string GetCurrentWindow() =>
-        DateTimeOffset.UtcNow.ToString("yyyy-MM");
+    private string GetCurrentWindow() =>
+        _timeProvider.GetUtcNow().ToString("yyyy-MM");
 
     private Task<ServiceTier> GetServiceTierAsync(string serviceId, CancellationToken ct)
     {

src/Policy/__Libraries/StellaOps.Policy/StellaOps.Policy.csproj

@@ -28,5 +28,6 @@
     <ProjectReference Include="../../../Attestor/__Libraries/StellaOps.Attestor.ProofChain/StellaOps.Attestor.ProofChain.csproj" />
     <ProjectReference Include="../../../__Libraries/StellaOps.Canonical.Json/StellaOps.Canonical.Json.csproj" />
     <ProjectReference Include="../../../__Libraries/StellaOps.Cryptography/StellaOps.Cryptography.csproj" />
+    <ProjectReference Include="../../../__Libraries/StellaOps.Determinism.Abstractions/StellaOps.Determinism.Abstractions.csproj" />
   </ItemGroup>
 </Project>