feat(health): SiteAuditWriteFailures counter + AuditLog bridge (#23)

Bundle G of Audit Log #23 M2. Bridges the FallbackAuditWriter primary-
failure counter into the Site Health Monitoring report payload so a
sustained audit-write outage surfaces on /monitoring/health instead of
disappearing into a NoOp sink.

- SiteHealthReport: add SiteAuditWriteFailures (defaulted, additive).
- ISiteHealthCollector + SiteHealthCollector: new
  IncrementSiteAuditWriteFailures() counter, per-interval reset
  semantics matching ScriptErrorCount / DeadLetterCount.
- HealthMetricsAuditWriteFailureCounter: adapter forwarding
  IAuditWriteFailureCounter.Increment() to the collector.
- AddAuditLogHealthMetricsBridge(): swaps the NoOp default
  registration for the real bridge; called from
  SiteServiceRegistration after AddSiteHealthMonitoring + AddAuditLog.
- Existing host-wiring test updated: site composition now resolves
  HealthMetricsAuditWriteFailureCounter (not NoOp).

Tests: HealthMonitoring 60 -> 63 (3 new), AuditLog 56 -> 59 (3 new),
full solution green.

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ScadaLink.AuditLog.Configuration;
@@ -103,4 +104,36 @@ public static class ServiceCollectionExtensions
 
         return services;
     }
+
+    /// <summary>
+    /// Audit Log (#23) M2 Bundle G — swap the default
+    /// <see cref="NoOpAuditWriteFailureCounter"/> registration for the real
+    /// <see cref="HealthMetricsAuditWriteFailureCounter"/> bridge so the
+    /// FallbackAuditWriter primary-failure counter surfaces in the site health
+    /// report payload as <c>SiteHealthReport.SiteAuditWriteFailures</c>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Must be called AFTER both <see cref="AddAuditLog"/> (registers the
+    /// NoOp default this method replaces) and
+    /// <c>ScadaLink.HealthMonitoring.ServiceCollectionExtensions.AddHealthMonitoring</c>
+    /// or <c>AddSiteHealthMonitoring</c> (registers the
+    /// <see cref="ISiteHealthCollector"/> the bridge depends on). Resolving
+    /// <see cref="IAuditWriteFailureCounter"/> without the latter throws
+    /// <see cref="InvalidOperationException"/> at <c>GetRequiredService</c>
+    /// time — by design, since a silent NoOp would mask a misconfiguration.
+    /// </para>
+    /// <para>
+    /// Idempotent — calling twice replaces the descriptor each time without
+    /// piling up registrations.
+    /// </para>
+    /// </remarks>
+    public static IServiceCollection AddAuditLogHealthMetricsBridge(this IServiceCollection services)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        services.Replace(
+            ServiceDescriptor.Singleton<IAuditWriteFailureCounter, HealthMetricsAuditWriteFailureCounter>());
+        return services;
+    }
 }