feat(health): SiteAuditWriteFailures counter + AuditLog bridge (#23)

Bundle G of Audit Log #23 M2. Bridges the FallbackAuditWriter primary-
failure counter into the Site Health Monitoring report payload so a
sustained audit-write outage surfaces on /monitoring/health instead of
disappearing into a NoOp sink.

- SiteHealthReport: add SiteAuditWriteFailures (defaulted, additive).
- ISiteHealthCollector + SiteHealthCollector: new
  IncrementSiteAuditWriteFailures() counter, per-interval reset
  semantics matching ScriptErrorCount / DeadLetterCount.
- HealthMetricsAuditWriteFailureCounter: adapter forwarding
  IAuditWriteFailureCounter.Increment() to the collector.
- AddAuditLogHealthMetricsBridge(): swaps the NoOp default
  registration for the real bridge; called from
  SiteServiceRegistration after AddSiteHealthMonitoring + AddAuditLog.
- Existing host-wiring test updated: site composition now resolves
  HealthMetricsAuditWriteFailureCounter (not NoOp).

Tests: HealthMonitoring 60 -> 63 (3 new), AuditLog 56 -> 59 (3 new),
full solution green.

tests/ScadaLink.SiteRuntime.Tests/Actors/DeploymentManagerRedeployTests.cs

@@ -69,6 +69,7 @@ public class DeploymentManagerRedeployTests : TestKit, IDisposable
         public void IncrementScriptError() { }
         public void IncrementAlarmError() { }
         public void IncrementDeadLetter() { }
+        public void IncrementSiteAuditWriteFailures() { }
         public void UpdateConnectionHealth(string connectionName, ConnectionHealth health) { }
         public void RemoveConnection(string connectionName) { }
         public void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved) { }