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.HealthMonitoring.Tests/SiteAuditWriteFailuresMetricTests.cs

@@ -0,0 +1,52 @@
+namespace ScadaLink.HealthMonitoring.Tests;
+
+/// <summary>
+/// Bundle G (M2-T11) regression coverage. The site-side Audit Log writer chain
+/// (FallbackAuditWriter) increments <see cref="IAuditWriteFailureCounter"/>
+/// every time the primary SQLite writer throws. Bundle G bridges that counter
+/// into the Site Health Monitoring report payload as <c>SiteAuditWriteFailures</c>
+/// so a sustained audit-write outage surfaces on /monitoring/health rather than
+/// disappearing into a NoOp sink.
+/// </summary>
+public class SiteAuditWriteFailuresMetricTests
+{
+    private readonly SiteHealthCollector _collector = new();
+
+    [Fact]
+    public void Increment_Three_Times_Counter_Reports_3()
+    {
+        _collector.IncrementSiteAuditWriteFailures();
+        _collector.IncrementSiteAuditWriteFailures();
+        _collector.IncrementSiteAuditWriteFailures();
+
+        var report = _collector.CollectReport("site-1");
+
+        Assert.Equal(3, report.SiteAuditWriteFailures);
+    }
+
+    [Fact]
+    public void Report_Payload_Includes_SiteAuditWriteFailures_AsZeroByDefault()
+    {
+        var report = _collector.CollectReport("site-1");
+
+        Assert.Equal(0, report.SiteAuditWriteFailures);
+    }
+
+    /// <summary>
+    /// Mirrors the existing per-interval reset semantics for ScriptErrorCount /
+    /// AlarmEvaluationErrorCount / DeadLetterCount — SiteAuditWriteFailures is an
+    /// interval count, not a running total.
+    /// </summary>
+    [Fact]
+    public void CollectReport_Resets_SiteAuditWriteFailures()
+    {
+        _collector.IncrementSiteAuditWriteFailures();
+        _collector.IncrementSiteAuditWriteFailures();
+
+        var first = _collector.CollectReport("site-1");
+        Assert.Equal(2, first.SiteAuditWriteFailures);
+
+        var second = _collector.CollectReport("site-1");
+        Assert.Equal(0, second.SiteAuditWriteFailures);
+    }
+}