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.

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -51,6 +51,13 @@ public static class SiteServiceRegistration
         // ScriptRuntimeContext, when Bundle F lands) reaches for.
         services.AddAuditLog(config);
 
+        // Audit Log (#23) M2 Bundle G — bridge FallbackAuditWriter primary
+        // failures into the site health report payload as
+        // SiteAuditWriteFailures. Must come AFTER both AddSiteHealthMonitoring
+        // (registers ISiteHealthCollector) and AddAuditLog (registers the
+        // NoOp default this call replaces).
+        services.AddAuditLogHealthMetricsBridge();
+
         // WP-13: Akka.NET bootstrap via hosted service
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());