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.AuditLog.Tests/Site/HealthMetricsAuditWriteFailureCounterTests.cs

@@ -0,0 +1,46 @@
+using NSubstitute;
+using ScadaLink.AuditLog.Site;
+using ScadaLink.HealthMonitoring;
+
+namespace ScadaLink.AuditLog.Tests.Site;
+
+/// <summary>
+/// Bundle G (M2-T11) — the <see cref="HealthMetricsAuditWriteFailureCounter"/>
+/// adapter is the production binding for <see cref="IAuditWriteFailureCounter"/>
+/// on site nodes; it forwards every FallbackAuditWriter primary failure into
+/// the shared <see cref="ISiteHealthCollector"/> so the site health report
+/// surfaces the failure count as <c>SiteAuditWriteFailures</c>.
+/// </summary>
+public class HealthMetricsAuditWriteFailureCounterTests
+{
+    [Fact]
+    public void Increment_Routes_To_Collector_IncrementSiteAuditWriteFailures()
+    {
+        var collector = Substitute.For<ISiteHealthCollector>();
+        var counter = new HealthMetricsAuditWriteFailureCounter(collector);
+
+        counter.Increment();
+
+        collector.Received(1).IncrementSiteAuditWriteFailures();
+    }
+
+    [Fact]
+    public void Increment_Multiple_Calls_Route_To_Collector_Each_Time()
+    {
+        var collector = Substitute.For<ISiteHealthCollector>();
+        var counter = new HealthMetricsAuditWriteFailureCounter(collector);
+
+        counter.Increment();
+        counter.Increment();
+        counter.Increment();
+
+        collector.Received(3).IncrementSiteAuditWriteFailures();
+    }
+
+    [Fact]
+    public void Construction_With_Null_Collector_Throws_ArgumentNullException()
+    {
+        Assert.Throws<ArgumentNullException>(
+            () => new HealthMetricsAuditWriteFailureCounter(null!));
+    }
+}