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.SiteEventLogging/ISiteEventLogger.cs

@@ -32,10 +32,9 @@ public interface ISiteEventLogger
     /// <summary>
     /// SiteEventLogging-018: total number of event writes that have failed
     /// (SQLite error, disk full, bounded-queue overflow drop, etc.) since this
-    /// logger was created. Available for future Health Monitoring integration —
-    /// promoted onto the interface so a Health consumer can read it without a
-    /// concrete-type downcast. Not yet polled by Health Monitoring; the wiring
-    /// is tracked separately.
+    /// logger was created. Polled by <c>SiteEventLogFailureCountReporter</c>
+    /// (HealthMonitoring — M2.16 / #30) every 30 s and surfaced on the site
+    /// health report as <c>SiteHealthReport.SiteEventLogWriteFailures</c>.
     /// </summary>
     long FailedWriteCount { get; }
 }