feat(host): register Audit Log #23 singletons with dedicated dispatcher (#23)

Wires Bundle E of the M2 site-sync pipeline:

- AddAuditLog extended to register the site writer chain (SqliteAuditWriter
  singleton + ISiteAuditQueue forward + RingBufferFallback + FallbackAuditWriter
  composing them) and the telemetry collaborators (SiteAuditTelemetryOptions,
  SqliteAuditWriterOptions, IAuditWriteFailureCounter NoOp default,
  ISiteStreamAuditClient NoOp default).
- AkkaHostedService central role: AuditLogIngestActor as ClusterSingletonManager
  (singleton name 'audit-log-ingest') + ClusterSingletonProxy, mirroring the
  Notification Outbox pattern. Proxy is offered to SiteStreamGrpcServer if it
  resolves (Site path only today; M6 reconciliation will host gRPC on central).
- AkkaHostedService site role: SiteAuditTelemetryActor (per-site, NOT a
  singleton because each site is its own cluster), bound to a dedicated
  audit-telemetry-dispatcher (ForkJoinDispatcher, 2 dedicated threads).
- Program.cs + SiteServiceRegistration.Configure call AddAuditLog on both roles.
- AuditLogIngestActor gains a second constructor that takes IServiceProvider so
  the cluster singleton can create a fresh scope per message — IAuditLogRepository
  is a scoped EF Core service and cannot be pre-resolved from the root. The
  IAuditLogRepository constructor remains for Bundle D's MSSQL-fixture tests.

NoOp ISiteStreamAuditClient is deliberate: no site→central gRPC channel exists
in M2 (sites talk to central via Akka ClusterClient; gRPC SiteStreamService is
hosted on sites for central→site streaming). M6 reconciliation introduces the
real gRPC site→central client + central-hosted gRPC server. Bundle H's
integration test substitutes a stub client directly via the actor's Props.

Tests:
- tests/ScadaLink.AuditLog.Tests/AddAuditLogTests.cs — 11 tests (was 3): writer
  singleton, IAuditWriter as FallbackAuditWriter, ISiteAuditQueue same-instance
  as SqliteAuditWriter, options bind round-trip, NoOp default assertions.
- tests/ScadaLink.Host.Tests/AkkaHostedServiceAuditWiringTests.cs (new) — 13
  tests: BuildHocon emits audit-telemetry-dispatcher block with the expected
  type/throughput/thread-count; Central composition root resolves the writer
  chain + options; Site composition root resolves the writer chain + options +
  NoOp client.

Verified: dotnet build clean, 23 test suites green (Host 194 + AuditLog 54).

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -1,3 +1,4 @@
+using ScadaLink.AuditLog;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Communication;
 using ScadaLink.DataConnectionLayer;
@@ -44,6 +45,12 @@ public static class SiteServiceRegistration
         services.AddStoreAndForward();
         services.AddSiteEventLogging();
 
+        // Audit Log (#23) — site-side hot-path writer + telemetry collaborators.
+        // The SiteAuditTelemetryActor itself is registered by AkkaHostedService
+        // in the site-role block; this call wires every DI dependency it (and
+        // ScriptRuntimeContext, when Bundle F lands) reaches for.
+        services.AddAuditLog(config);
+
         // WP-13: Akka.NET bootstrap via hosted service
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());