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/ServiceCollectionExtensions.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ScadaLink.AuditLog.Configuration;
@@ -103,4 +104,36 @@ public static class ServiceCollectionExtensions
 
         return services;
     }
+
+    /// <summary>
+    /// Audit Log (#23) M2 Bundle G — swap the default
+    /// <see cref="NoOpAuditWriteFailureCounter"/> registration for the real
+    /// <see cref="HealthMetricsAuditWriteFailureCounter"/> bridge so the
+    /// FallbackAuditWriter primary-failure counter surfaces in the site health
+    /// report payload as <c>SiteHealthReport.SiteAuditWriteFailures</c>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Must be called AFTER both <see cref="AddAuditLog"/> (registers the
+    /// NoOp default this method replaces) and
+    /// <c>ScadaLink.HealthMonitoring.ServiceCollectionExtensions.AddHealthMonitoring</c>
+    /// or <c>AddSiteHealthMonitoring</c> (registers the
+    /// <see cref="ISiteHealthCollector"/> the bridge depends on). Resolving
+    /// <see cref="IAuditWriteFailureCounter"/> without the latter throws
+    /// <see cref="InvalidOperationException"/> at <c>GetRequiredService</c>
+    /// time — by design, since a silent NoOp would mask a misconfiguration.
+    /// </para>
+    /// <para>
+    /// Idempotent — calling twice replaces the descriptor each time without
+    /// piling up registrations.
+    /// </para>
+    /// </remarks>
+    public static IServiceCollection AddAuditLogHealthMetricsBridge(this IServiceCollection services)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        services.Replace(
+            ServiceDescriptor.Singleton<IAuditWriteFailureCounter, HealthMetricsAuditWriteFailureCounter>());
+        return services;
+    }
 }

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();
+}

src/ScadaLink.Commons/Messages/Health/SiteHealthReport.cs

@@ -20,7 +20,12 @@ public record SiteHealthReport(
     IReadOnlyDictionary<string, string>? DataConnectionEndpoints = null,
     IReadOnlyDictionary<string, TagQualityCounts>? DataConnectionTagQuality = null,
     int ParkedMessageCount = 0,
-    IReadOnlyList<NodeStatus>? ClusterNodes = null);
+    IReadOnlyList<NodeStatus>? ClusterNodes = null,
+    // Audit Log (#23) M2 Bundle G: per-interval count of FallbackAuditWriter
+    // primary failures (SQLite throws routed to the drop-oldest ring). Surfaces
+    // a sustained audit-write outage on /monitoring/health. Defaults to 0 so
+    // existing producers / tests that don't construct the field stay valid.
+    int SiteAuditWriteFailures = 0);
 
 /// <summary>
 /// Broadcast wrapper used between central nodes to keep per-node

src/ScadaLink.HealthMonitoring/ISiteHealthCollector.cs

@@ -12,6 +12,13 @@ public interface ISiteHealthCollector
     void IncrementScriptError();
     void IncrementAlarmError();
     void IncrementDeadLetter();
+    /// <summary>
+    /// Audit Log (#23) Bundle G — increment the per-interval count of
+    /// <c>FallbackAuditWriter</c> primary failures. Bridged from the
+    /// <c>IAuditWriteFailureCounter</c> binding registered via
+    /// <c>AddAuditLogHealthMetricsBridge()</c>.
+    /// </summary>
+    void IncrementSiteAuditWriteFailures();
     void UpdateConnectionHealth(string connectionName, ConnectionHealth health);
     void RemoveConnection(string connectionName);
     void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved);

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -13,6 +13,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _scriptErrorCount;
     private int _alarmErrorCount;
     private int _deadLetterCount;
+    private int _siteAuditWriteFailures;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private readonly ConcurrentDictionary<string, string> _connectionEndpoints = new();
@@ -61,6 +62,18 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Increment(ref _deadLetterCount);
     }
 
+    /// <summary>
+    /// Audit Log (#23) Bundle G — increment the per-interval count of
+    /// <c>FallbackAuditWriter</c> primary failures. Bridged from the
+    /// <c>IAuditWriteFailureCounter</c> binding registered via
+    /// <c>AddAuditLogHealthMetricsBridge()</c>; reset every interval together
+    /// with the other per-interval counters.
+    /// </summary>
+    public void IncrementSiteAuditWriteFailures()
+    {
+        Interlocked.Increment(ref _siteAuditWriteFailures);
+    }
+
     /// <summary>
     /// Update the health status for a named data connection.
     /// Called by DCL when connection state changes.
@@ -144,6 +157,7 @@ public class SiteHealthCollector : ISiteHealthCollector
         var scriptErrors = Interlocked.Exchange(ref _scriptErrorCount, 0);
         var alarmErrors = Interlocked.Exchange(ref _alarmErrorCount, 0);
         var deadLetters = Interlocked.Exchange(ref _deadLetterCount, 0);
+        var siteAuditWriteFailures = Interlocked.Exchange(ref _siteAuditWriteFailures, 0);
 
         // Snapshot current connection and tag resolution state
         var connectionStatuses = new Dictionary<string, ConnectionHealth>(_connectionStatuses);
@@ -175,6 +189,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             DataConnectionEndpoints: connectionEndpoints,
             DataConnectionTagQuality: tagQuality,
             ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0),
-            ClusterNodes: _clusterNodes?.ToList());
+            ClusterNodes: _clusterNodes?.ToList(),
+            SiteAuditWriteFailures: siteAuditWriteFailures);
     }
 }

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -51,6 +51,13 @@ public static class SiteServiceRegistration
         // ScriptRuntimeContext, when Bundle F lands) reaches for.
         services.AddAuditLog(config);
 
+        // Audit Log (#23) M2 Bundle G — bridge FallbackAuditWriter primary
+        // failures into the site health report payload as
+        // SiteAuditWriteFailures. Must come AFTER both AddSiteHealthMonitoring
+        // (registers ISiteHealthCollector) and AddAuditLog (registers the
+        // NoOp default this call replaces).
+        services.AddAuditLogHealthMetricsBridge();
+
         // WP-13: Akka.NET bootstrap via hosted service
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());