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/ServiceCollectionExtensions.cs

@@ -1,5 +1,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 
 namespace ZB.MOM.WW.ScadaBridge.HealthMonitoring;
@@ -50,6 +52,68 @@ public static class ServiceCollectionExtensions
         return services;
     }
 
+    /// <summary>
+    /// Site Event Logging (#12) M2.16 (#30) — register the
+    /// <see cref="SiteEventLogFailureCountReporter"/> hosted service that
+    /// periodically reads the cumulative event-log write-failure count and
+    /// pushes it into <see cref="ISiteHealthCollector"/> as a point-in-time
+    /// snapshot (<c>SiteEventLogWriteFailures</c> on the site health report).
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Must be called AFTER <see cref="AddSiteHealthMonitoring"/> (or
+    /// <see cref="AddHealthMonitoring"/>) which registers the
+    /// <see cref="ISiteHealthCollector"/> the reporter depends on.
+    /// </para>
+    /// <para>
+    /// <b>Why a Func&lt;long&gt; delegate instead of ISiteEventLogger.</b>
+    /// <c>HealthMonitoring</c> must not reference <c>SiteEventLogging</c> directly —
+    /// the <c>StoreAndForward → SiteEventLogging</c> edge already exists in the
+    /// transitive graph, and <c>HealthMonitoring → StoreAndForward</c> is an
+    /// existing direct reference; adding <c>HealthMonitoring → SiteEventLogging</c>
+    /// would complete a cycle. The <see cref="Func{TResult}"/> delegate seam keeps
+    /// the dependency acyclic: the caller (Host site wiring) captures
+    /// <c>ISiteEventLogger.FailedWriteCount</c> as a lambda and passes it here.
+    /// </para>
+    /// <para>
+    /// Idempotent — a sentinel check on the
+    /// <see cref="SiteEventLogFailureCountReporter"/> hosted-service descriptor
+    /// short-circuits subsequent calls so the hosted service is not
+    /// double-registered (AddHostedService has no TryAdd variant).
+    /// </para>
+    /// </remarks>
+    /// <param name="services">The service collection to register into.</param>
+    /// <param name="failedWriteCountProvider">
+    /// A factory delegate that, given the root <see cref="IServiceProvider"/>,
+    /// returns a <see cref="Func{TResult}"/> that reads the current cumulative
+    /// event-log write-failure count. Typically:
+    /// <c>sp => () => sp.GetRequiredService&lt;ISiteEventLogger&gt;().FailedWriteCount</c>.
+    /// The factory is evaluated once at hosted-service resolution time; the inner
+    /// <see cref="Func{TResult}"/> is called on every poll tick.
+    /// </param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
+    public static IServiceCollection AddSiteEventLogHealthMetricsBridge(
+        this IServiceCollection services,
+        Func<IServiceProvider, Func<long>> failedWriteCountProvider)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        ArgumentNullException.ThrowIfNull(failedWriteCountProvider);
+
+        // Idempotent guard — mirrors AddAuditLogHealthMetricsBridge's
+        // SiteAuditBacklogReporter sentinel check.
+        if (services.Any(d => d.ImplementationType == typeof(SiteEventLogFailureCountReporter)))
+        {
+            return services;
+        }
+
+        services.AddHostedService(sp => new SiteEventLogFailureCountReporter(
+            failedWriteCountProvider(sp),
+            sp.GetRequiredService<ISiteHealthCollector>(),
+            sp.GetRequiredService<ILogger<SiteEventLogFailureCountReporter>>()));
+
+        return services;
+    }
+
     /// <summary>
     /// HealthMonitoring-014: register the <see cref="HealthMonitoringOptionsValidator"/>
     /// so a misconfigured <c>ScadaBridge:HealthMonitoring</c> section (zero/negative