feat(notif): NotificationOutboxActor + CentralAuditWriter wired (#23 M4)

M4 Bundle B (B1) — add the central-only ICentralAuditWriter implementation
and inject it into NotificationOutboxActor so subsequent tasks (B2/B3) can
route attempt + terminal lifecycle events through the direct-write audit path.

- CentralAuditWriter: thin wrapper around IAuditLogRepository.InsertIfNotExistsAsync;
  scope-per-call (matches AuditLogIngestActor / NotificationOutboxActor pattern);
  stamps IngestedAtUtc; swallows all internal failures (alog.md §13).
- Registered as a singleton in AddAuditLog.
- NotificationOutboxActor ctor takes ICentralAuditWriter (validated non-null).
- Host wiring resolves the writer once from the root provider and passes it
  into the singleton's Props.Create call.
- Existing TestKit fixtures updated with a NoOpCentralAuditWriter helper so
  tests that don't exercise audit emission still compile and pass.

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -3,6 +3,7 @@ using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
+using ScadaLink.AuditLog.Central;
 using ScadaLink.AuditLog.Configuration;
 using ScadaLink.AuditLog.Site;
 using ScadaLink.AuditLog.Site.Telemetry;
@@ -129,6 +130,17 @@ public static class ServiceCollectionExtensions
         services.AddSingleton<ICachedCallLifecycleObserver>(
             sp => sp.GetRequiredService<CachedCallLifecycleBridge>());
 
+        // M4 Bundle B: central direct-write audit writer used by
+        // NotificationOutboxActor (Bundle B) and Inbound API (Bundle C/D) to
+        // emit AuditLog rows that originate ON central, not via site telemetry.
+        // Singleton — the writer is stateless; its per-call scope opens a fresh
+        // IAuditLogRepository (a SCOPED EF Core service registered by
+        // ScadaLink.ConfigurationDatabase). The interface (ICentralAuditWriter)
+        // is intentionally distinct from IAuditWriter so site composition roots
+        // do not accidentally bind it; central composition roots that include
+        // AddConfigurationDatabase get a working implementation transparently.
+        services.AddSingleton<ICentralAuditWriter, CentralAuditWriter>();
+
         return services;
     }