feat(health): wire ISiteEventLogger.FailedWriteCount into SiteHealthReport (#30, M2.16)

Add SiteHealthReport.SiteEventLogWriteFailures (trailing optional long = 0,
additive-only), ISiteHealthCollector.SetSiteEventLogWriteFailures (default
no-op so existing fakes compile), and SiteEventLogFailureCountReporter
(hosted service in HealthMonitoring, Func<long> delegate to avoid the
HealthMonitoring → StoreAndForward → SiteEventLogging cycle).

Registration helper AddSiteEventLogHealthMetricsBridge added to
HealthMonitoring.ServiceCollectionExtensions; wired in
SiteServiceRegistration after AddSiteEventLogging.

Tests: SiteEventLogWriteFailuresMetricTests (4 collector tests) +
SiteEventLogFailureCountReporterTests (2 poller tests) in
HealthMonitoring.Tests. 79/79 HealthMonitoring.Tests green,
59/59 SiteEventLogging.Tests green, 0 warnings.

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/SiteHealthCollector.cs

@@ -17,6 +17,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _siteAuditWriteFailures;
     private int _auditRedactionFailures;
     private volatile SiteAuditBacklogSnapshot? _siteAuditBacklog;
+    private long _siteEventLogWriteFailures;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private readonly ConcurrentDictionary<string, string> _connectionEndpoints = new();
@@ -77,6 +78,12 @@ public class SiteHealthCollector : ISiteHealthCollector
         _siteAuditBacklog = snapshot ?? throw new ArgumentNullException(nameof(snapshot));
     }
 
+    /// <inheritdoc />
+    public void SetSiteEventLogWriteFailures(long count)
+    {
+        Interlocked.Exchange(ref _siteEventLogWriteFailures, count);
+    }
+
     /// <inheritdoc />
     public void UpdateConnectionHealth(string connectionName, ConnectionHealth health)
     {
@@ -206,6 +213,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             ClusterNodes: _clusterNodes?.ToList(),
             SiteAuditWriteFailures: siteAuditWriteFailures,
             AuditRedactionFailure: auditRedactionFailures,
-            SiteAuditBacklog: _siteAuditBacklog);
+            SiteAuditBacklog: _siteAuditBacklog,
+            SiteEventLogWriteFailures: Interlocked.Read(ref _siteEventLogWriteFailures));
     }
 }