feat(host): register SiteCallAuditActor + CachedCallTelemetry forwarder/bridge (#22, #23 M3)

M3 Bundle F (Task F1) wires the cached-call audit pipeline through the
composition roots:

- Central: register SiteCallAuditActor as a cluster singleton + proxy
  (mirrors AuditLogIngestActor and NotificationOutboxActor). Program.cs
  calls .AddSiteCallAudit() on the central role.
- Site: register ICachedCallTelemetryForwarder + CachedCallLifecycleBridge
  in AddAuditLog (lazy factory — Central nodes degrade to audit-only
  emission because IOperationTrackingStore is site-only).
- Site: bind CachedCallLifecycleBridge to ICachedCallLifecycleObserver so
  StoreAndForwardService picks it up via DI.
- Site: introduce IStoreAndForwardSiteContext + Host adapter to surface the
  site id to StoreAndForwardService without creating a
  StoreAndForward -> HealthMonitoring project-reference cycle.
- ScriptExecutionActor resolves ICachedCallTelemetryForwarder per script
  scope and threads it into ScriptRuntimeContext.

CachedCallTelemetryForwarder's IOperationTrackingStore dependency is now
nullable so Central DI validation succeeds with the lazy registration; the
forwarder's tracking-half emission is a no-op when the store is absent.

Tests:
- AkkaHostedServiceAuditWiringTests: Central host builds with
  AddSiteCallAudit and resolves ICachedCallTelemetryForwarder; Site
  resolves the forwarder + bridge + observer + IStoreAndForwardSiteContext.
- Full solution: 194 Host tests green, 241 SiteRuntime tests green, every
  other suite unchanged.

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -342,6 +342,35 @@ akka {{
             "AuditLogIngestActor singleton created (gRPC server bound: {GrpcBound})",
             grpcServer is not null);
 
+        // Site Call Audit (#22) — central singleton mirrors the AuditLogIngest
+        // and NotificationOutbox patterns. M3's dual-write transaction routes
+        // SiteCalls upserts through AuditLogIngestActor's own scope-per-message
+        // ISiteCallAuditRepository resolution, so this singleton is not on the
+        // M3 happy-path hot path; it exists so future direct-write callers
+        // (reconciliation puller, central→site Retry/Discard relay, KPI
+        // projector) Ask through a stable cluster proxy without further wiring.
+        // Like AuditLogIngestActor, the actor takes the root IServiceProvider
+        // and creates a fresh scope per message because ISiteCallAuditRepository
+        // is a scoped EF Core service.
+        var siteCallAuditLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
+            .CreateLogger<ScadaLink.SiteCallAudit.SiteCallAuditActor>();
+
+        var siteCallAuditSingletonProps = ClusterSingletonManager.Props(
+            singletonProps: Props.Create(() => new ScadaLink.SiteCallAudit.SiteCallAuditActor(
+                _serviceProvider,
+                siteCallAuditLogger)),
+            terminationMessage: PoisonPill.Instance,
+            settings: ClusterSingletonManagerSettings.Create(_actorSystem!)
+                .WithSingletonName("site-call-audit"));
+        _actorSystem!.ActorOf(siteCallAuditSingletonProps, "site-call-audit-singleton");
+
+        var siteCallAuditProxyProps = ClusterSingletonProxy.Props(
+            singletonManagerPath: "/user/site-call-audit-singleton",
+            settings: ClusterSingletonProxySettings.Create(_actorSystem)
+                .WithSingletonName("site-call-audit"));
+        _actorSystem.ActorOf(siteCallAuditProxyProps, "site-call-audit-proxy");
+        _logger.LogInformation("SiteCallAuditActor singleton created");
+
         _logger.LogInformation("Central actors registered. CentralCommunicationActor created.");
     }