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);

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -13,6 +13,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _scriptErrorCount;
     private int _alarmErrorCount;
     private int _deadLetterCount;
+    private int _siteAuditWriteFailures;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private readonly ConcurrentDictionary<string, string> _connectionEndpoints = new();
@@ -61,6 +62,18 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Increment(ref _deadLetterCount);
     }
 
+    /// <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>; reset every interval together
+    /// with the other per-interval counters.
+    /// </summary>
+    public void IncrementSiteAuditWriteFailures()
+    {
+        Interlocked.Increment(ref _siteAuditWriteFailures);
+    }
+
     /// <summary>
     /// Update the health status for a named data connection.
     /// Called by DCL when connection state changes.
@@ -144,6 +157,7 @@ public class SiteHealthCollector : ISiteHealthCollector
         var scriptErrors = Interlocked.Exchange(ref _scriptErrorCount, 0);
         var alarmErrors = Interlocked.Exchange(ref _alarmErrorCount, 0);
         var deadLetters = Interlocked.Exchange(ref _deadLetterCount, 0);
+        var siteAuditWriteFailures = Interlocked.Exchange(ref _siteAuditWriteFailures, 0);
 
         // Snapshot current connection and tag resolution state
         var connectionStatuses = new Dictionary<string, ConnectionHealth>(_connectionStatuses);
@@ -175,6 +189,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             DataConnectionEndpoints: connectionEndpoints,
             DataConnectionTagQuality: tagQuality,
             ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0),
-            ClusterNodes: _clusterNodes?.ToList());
+            ClusterNodes: _clusterNodes?.ToList(),
+            SiteAuditWriteFailures: siteAuditWriteFailures);
     }
 }