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.

tests/ScadaLink.Host.Tests/AkkaHostedServiceAuditWiringTests.cs

@@ -274,11 +274,16 @@ public class SiteAuditWiringTests : IDisposable
     }
 
     [Fact]
-    public void Site_Resolves_IAuditWriteFailureCounter_AsNoOpDefault()
+    public void Site_Resolves_IAuditWriteFailureCounter_AsHealthMetricsBridge()
     {
+        // Bundle G (M2-T11): site composition root calls
+        // AddAuditLogHealthMetricsBridge() after AddAuditLog + AddSiteHealthMonitoring,
+        // which swaps the NoOp default for the real health-metrics bridge so
+        // FallbackAuditWriter primary failures surface in the site health
+        // report payload as SiteAuditWriteFailures.
         var counter = _host.Services.GetService<IAuditWriteFailureCounter>();
         Assert.NotNull(counter);
-        Assert.IsType<NoOpAuditWriteFailureCounter>(counter);
+        Assert.IsType<HealthMetricsAuditWriteFailureCounter>(counter);
     }
 
     [Fact]