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.Host/SiteServiceRegistration.cs

@@ -58,6 +58,16 @@ public static class SiteServiceRegistration
         services.AddStoreAndForward();
         services.AddSiteEventLogging();
 
+        // Site Event Logging (#12) M2.16 (#30) — bridge ISiteEventLogger.FailedWriteCount
+        // into the site health report as a point-in-time SiteEventLogWriteFailures field.
+        // Must come AFTER both AddSiteHealthMonitoring (registers ISiteHealthCollector) and
+        // AddSiteEventLogging (registers ISiteEventLogger). The outer Func<IServiceProvider, …>
+        // is evaluated once at hosted-service resolution time (root IServiceProvider is available);
+        // the inner Func<long> is called on every poll tick and reads FailedWriteCount from the
+        // already-resolved ISiteEventLogger singleton.
+        services.AddSiteEventLogHealthMetricsBridge(
+            sp => () => sp.GetRequiredService<ISiteEventLogger>().FailedWriteCount);
+
         // Audit Log (#23) — site-side hot-path writer + telemetry collaborators.
         // The SiteAuditTelemetryActor itself is registered by AkkaHostedService
         // in the site-role block; this call wires every DI dependency it (and