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/AddAuditLogTests.cs

@@ -7,6 +7,7 @@ using ScadaLink.AuditLog.Configuration;
 using ScadaLink.AuditLog.Site;
 using ScadaLink.AuditLog.Site.Telemetry;
 using ScadaLink.Commons.Interfaces.Services;
+using ScadaLink.HealthMonitoring;
 
 namespace ScadaLink.AuditLog.Tests;
 
@@ -187,4 +188,56 @@ public class AddAuditLogTests
         Assert.Equal(3, opts.BusyIntervalSeconds);
         Assert.Equal(60, opts.IdleIntervalSeconds);
     }
+
+    // -- Bundle G (M2 Task G1) Site Health Monitoring bridge ----------------
+
+    [Fact]
+    public void AddAuditLogHealthMetricsBridge_Swaps_FailureCounter_To_HealthMetrics_Implementation()
+    {
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["AuditLog:SiteWriter:DatabasePath"] = ":memory:",
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
+        services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        services.AddAuditLog(config);
+        // The bridge depends on ISiteHealthCollector; AddHealthMonitoring is
+        // what registers it on the site (and the central self-host).
+        services.AddHealthMonitoring();
+        services.AddAuditLogHealthMetricsBridge();
+        using var provider = services.BuildServiceProvider();
+
+        var counter = provider.GetRequiredService<IAuditWriteFailureCounter>();
+
+        Assert.IsType<HealthMetricsAuditWriteFailureCounter>(counter);
+    }
+
+    [Fact]
+    public void AddAuditLogHealthMetricsBridge_Without_HealthMonitoring_Still_Resolves_But_Errors_On_Use()
+    {
+        // The bridge replaces the registration unconditionally; resolving the
+        // counter when ISiteHealthCollector is missing throws at GetRequiredService
+        // time. This documents the contract — callers must register
+        // AddHealthMonitoring() before the bridge.
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["AuditLog:SiteWriter:DatabasePath"] = ":memory:",
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
+        services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        services.AddAuditLog(config);
+        services.AddAuditLogHealthMetricsBridge();
+        using var provider = services.BuildServiceProvider();
+
+        Assert.Throws<InvalidOperationException>(
+            () => provider.GetRequiredService<IAuditWriteFailureCounter>());
+    }
 }

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!));
+    }
+}