fix(health): decouple AuditCentralHealthSnapshot from ActorSystem (#23 M6)

The snapshot's per-site stalled latch now lives on the snapshot itself
and is fed by SiteAuditTelemetryStalledTracker via ApplyStalled, removing
the chain that required ActorSystem at DI composition time. The tracker
is now constructed by AkkaHostedService once ActorSystem.Create returns,
with a lock-guarded auxiliary-disposable list so concurrent host
start/stop in tests cannot race the enumeration.

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -269,17 +269,6 @@ public static class ServiceCollectionExtensions
             .Bind(config.GetSection(PartitionMaintenanceSectionName));
         services.AddHostedService<AuditLogPartitionMaintenanceService>();
 
-        // M6 Bundle E (T7): central-singleton tracker for per-site
-        // SiteAuditTelemetryStalledChanged events published by
-        // SiteAuditReconciliationActor. Singleton so the EventStream
-        // subscription registers exactly once on host startup; the tracker's
-        // ctor needs an ActorSystem which is composed by the AkkaHostedService
-        // — we go through a factory so the SP is the source of truth for the
-        // system reference rather than re-resolving it on the consumer side.
-        services.AddSingleton<SiteAuditTelemetryStalledTracker>(sp =>
-            new SiteAuditTelemetryStalledTracker(
-                sp.GetRequiredService<Akka.Actor.ActorSystem>()));
-
         // M6 Bundle E (T8 + T9): central health snapshot — a single object
         // that owns the CentralAuditWriteFailures + AuditRedactionFailure
         // Interlocked counters AND surfaces them on
@@ -289,6 +278,11 @@ public static class ServiceCollectionExtensions
         // routes into the shared counters; site nodes keep their existing
         // Site bridges (registered by AddAuditLogHealthMetricsBridge) so
         // the same counter type does not shadow the site-side metric.
+        // The snapshot itself has no actor-system dependency — the
+        // per-site stalled latch is fed by SiteAuditTelemetryStalledTracker
+        // which the Akka bootstrap wires up after ActorSystem.Create returns
+        // (the tracker is NOT registered here because its construction
+        // requires ActorSystem, which is not a DI-resolvable singleton).
         services.AddSingleton<AuditCentralHealthSnapshot>();
         services.AddSingleton<IAuditCentralHealthSnapshot>(
             sp => sp.GetRequiredService<AuditCentralHealthSnapshot>());