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.HealthMonitoring/ISiteHealthCollector.cs

@@ -12,6 +12,13 @@ public interface ISiteHealthCollector
     void IncrementScriptError();
     void IncrementAlarmError();
     void IncrementDeadLetter();
+    /// <summary>
+    /// Audit Log (#23) Bundle G — increment the per-interval count of
+    /// <c>FallbackAuditWriter</c> primary failures. Bridged from the
+    /// <c>IAuditWriteFailureCounter</c> binding registered via
+    /// <c>AddAuditLogHealthMetricsBridge()</c>.
+    /// </summary>
+    void IncrementSiteAuditWriteFailures();
     void UpdateConnectionHealth(string connectionName, ConnectionHealth health);
     void RemoveConnection(string connectionName);
     void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved);