feat: Implement IsolatedReplayContext for deterministic audit replay

- Added IsolatedReplayContext class to provide an isolated environment for replaying audit bundles without external calls.
- Introduced methods for initializing the context, verifying input digests, and extracting inputs for policy evaluation.
- Created supporting interfaces and options for context configuration.

feat: Create ReplayExecutor for executing policy re-evaluation and verdict comparison

- Developed ReplayExecutor class to handle the execution of replay processes, including input verification and verdict comparison.
- Implemented detailed drift detection and error handling during replay execution.
- Added interfaces for policy evaluation and replay execution options.

feat: Add ScanSnapshotFetcher for fetching scan data and snapshots

- Introduced ScanSnapshotFetcher class to retrieve necessary scan data and snapshots for audit bundle creation.
- Implemented methods to fetch scan metadata, advisory feeds, policy snapshots, and VEX statements.
- Created supporting interfaces for scan data, feed snapshots, and policy snapshots.

src/Policy/StellaOps.Policy.Gateway/Program.cs

@@ -19,6 +19,8 @@ using StellaOps.Policy.Gateway.Endpoints;
 using StellaOps.Policy.Gateway.Infrastructure;
 using StellaOps.Policy.Gateway.Options;
 using StellaOps.Policy.Gateway.Services;
+using StellaOps.Policy.Deltas;
+using StellaOps.Policy.Snapshots;
 using StellaOps.Policy.Storage.Postgres;
 using Polly;
 using Polly.Extensions.Http;
@@ -119,6 +121,12 @@ builder.Services.AddScoped<IApprovalWorkflowService, ApprovalWorkflowService>();
 builder.Services.AddSingleton<IExceptionNotificationService, NoOpExceptionNotificationService>();
 builder.Services.AddHostedService<ExceptionExpiryWorker>();
 
+// Delta services
+builder.Services.AddScoped<IDeltaComputer, DeltaComputer>();
+builder.Services.AddScoped<IBaselineSelector, BaselineSelector>();
+builder.Services.AddScoped<ISnapshotStore, InMemorySnapshotStore>();
+builder.Services.AddScoped<StellaOps.Policy.Deltas.ISnapshotService, DeltaSnapshotServiceAdapter>();
+
 builder.Services.AddStellaOpsResourceServerAuthentication(
     builder.Configuration,
     configurationSection: $"{PolicyGatewayOptions.SectionName}:ResourceServer");
@@ -486,6 +494,9 @@ cvss.MapGet("/policies", async Task<IResult>(
 // Exception management endpoints
 app.MapExceptionEndpoints();
 
+// Delta management endpoints
+app.MapDeltasEndpoints();
+
 app.Run();
 
 static IAsyncPolicy<HttpResponseMessage> CreateAuthorityRetryPolicy(IServiceProvider provider)