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/ISiteHealthCollector.cs

@@ -111,6 +111,23 @@ public interface ISiteHealthCollector
     /// <param name="count">The number of parked messages.</param>
     void SetParkedMessageCount(int count);
 
+    /// <summary>
+    /// Site Event Logging (#12) M2.16 (#30) — replace the latest cumulative
+    /// site-event-log write-failure count (SQLite error, disk full,
+    /// bounded-queue overflow drop) used by the next <see cref="CollectReport"/>
+    /// call. Refreshed periodically by the <c>SiteEventLogFailureCountReporter</c>
+    /// hosted service. Point-in-time: the value is NOT reset on
+    /// <see cref="CollectReport"/>; it carries forward until the next poller
+    /// refresh. Default interface implementation is a no-op so existing test
+    /// fakes continue to compile without per-fake updates.
+    /// </summary>
+    /// <param name="count">The cumulative failed-write count from <c>ISiteEventLogger.FailedWriteCount</c>.</param>
+    void SetSiteEventLogWriteFailures(long count)
+    {
+        // Default no-op so test fakes do not need to be updated. The real
+        // SiteHealthCollector overrides this with the Interlocked.Exchange store.
+    }
+
     /// <summary>
     /// Sets the hostname of this node.
     /// </summary>