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.

tests/ScadaLink.NotificationOutbox.Tests/TestSupport/NoOpCentralAuditWriter.cs

@@ -0,0 +1,15 @@
+using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Interfaces.Services;
+
+namespace ScadaLink.NotificationOutbox.Tests.TestSupport;
+
+/// <summary>
+/// Test-only no-op <see cref="ICentralAuditWriter"/>. Used by existing
+/// NotificationOutboxActor TestKit fixtures whose tests pre-date the M4 Bundle B
+/// audit-writer injection — they don't care about audit emission, they just
+/// need a non-null collaborator so the actor's constructor succeeds.
+/// </summary>
+internal sealed class NoOpCentralAuditWriter : ICentralAuditWriter
+{
+    public Task WriteAsync(AuditEvent evt, CancellationToken ct = default) => Task.CompletedTask;
+}