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/Site/HealthMetricsAuditWriteFailureCounter.cs

@@ -0,0 +1,33 @@
+using ScadaLink.HealthMonitoring;
+
+namespace ScadaLink.AuditLog.Site;
+
+/// <summary>
+/// Audit Log (#23) M2 Bundle G — bridges <see cref="IAuditWriteFailureCounter"/>
+/// (incremented by <see cref="FallbackAuditWriter"/> every time the primary
+/// SQLite writer throws) into <see cref="ISiteHealthCollector"/> so the count
+/// surfaces in the site health report payload as
+/// <c>SiteHealthReport.SiteAuditWriteFailures</c>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Registered by <see cref="ServiceCollectionExtensions.AddAuditLogHealthMetricsBridge"/>;
+/// callers must register <c>AddHealthMonitoring()</c> first so
+/// <see cref="ISiteHealthCollector"/> resolves. The default <see cref="AddAuditLog"/>
+/// registration keeps <see cref="NoOpAuditWriteFailureCounter"/> for nodes
+/// where Site Health Monitoring is not wired (the silent-sink contract — audit
+/// write failures must NEVER abort the user-facing action, alog.md §7).
+/// </para>
+/// </remarks>
+public sealed class HealthMetricsAuditWriteFailureCounter : IAuditWriteFailureCounter
+{
+    private readonly ISiteHealthCollector _collector;
+
+    public HealthMetricsAuditWriteFailureCounter(ISiteHealthCollector collector)
+    {
+        _collector = collector ?? throw new ArgumentNullException(nameof(collector));
+    }
+
+    /// <inheritdoc/>
+    public void Increment() => _collector.IncrementSiteAuditWriteFailures();
+}