Phase 6.1 Stream C — health endpoints on :4841 + LogContextEnricher + Serilog JSON sink + CapabilityInvoker enrichment

Closes Stream C per docs/v2/implementation/phase-6-1-resilience-and-observability.md.

Core.Observability (new namespace):
- DriverHealthReport — pure-function aggregation over DriverHealthSnapshot list.
  Empty fleet = Healthy. Any Faulted = Faulted. Any Unknown/Initializing (no
  Faulted) = NotReady. Any Degraded or Reconnecting (no Faulted, no NotReady)
  = Degraded. Else Healthy. HttpStatus(verdict) maps to the Stream C.1 state
  matrix: Healthy/Degraded → 200, NotReady/Faulted → 503.
- LogContextEnricher — Serilog LogContext wrapper. Push(id, type, capability,
  correlationId) returns an IDisposable scope; inner log calls carry
  DriverInstanceId / DriverType / CapabilityName / CorrelationId structured
  properties automatically. NewCorrelationId = 12-hex-char GUID slice for
  cases where no OPC UA RequestHeader.RequestHandle is in flight.

CapabilityInvoker — now threads LogContextEnricher around every ExecuteAsync /
ExecuteWriteAsync call site. OtOpcUaServer passes driver.DriverType through
so logs correlate to the driver type too. Every capability call emits
structured fields per the Stream C.4 compliance check.

Server.Observability:
- HealthEndpointsHost — standalone HttpListener on http://localhost:4841/
  (loopback avoids Windows URL-ACL elevation; remote probing via reverse
  proxy or explicit netsh urlacl grant). Routes:
    /healthz → 200 when (configDbReachable OR usingStaleConfig); 503 otherwise.
      Body: status, uptimeSeconds, configDbReachable, usingStaleConfig.
    /readyz  → DriverHealthReport.Aggregate + HttpStatus mapping.
      Body: verdict, drivers[], degradedDrivers[], uptimeSeconds.
    anything else → 404.
  Disposal cooperative with the HttpListener shutdown.
- OpcUaApplicationHost starts the health host after the OPC UA server comes up
  and disposes it on shutdown. New OpcUaServerOptions knobs:
  HealthEndpointsEnabled (default true), HealthEndpointsPrefix (default
  http://localhost:4841/).

Program.cs:
- Serilog pipeline adds Enrich.FromLogContext + opt-in JSON file sink via
  `Serilog:WriteJson = true` appsetting. Uses Serilog.Formatting.Compact's
  CompactJsonFormatter (one JSON object per line — SIEMs like Splunk,
  Datadog, Graylog ingest without a regex parser).

Server.Tests:
- Existing 3 OpcUaApplicationHost integration tests now set
  HealthEndpointsEnabled=false to avoid port :4841 collisions under parallel
  execution.
- New HealthEndpointsHostTests (9): /healthz healthy empty fleet; stale-config
  returns 200 with flag; unreachable+no-cache returns 503; /readyz empty/
  Healthy/Faulted/Degraded/Initializing drivers return correct status and
  bodies; unknown path → 404. Uses ephemeral ports via Interlocked counter.

Core.Tests:
- DriverHealthReportTests (8): empty fleet, all-healthy, any-Faulted trumps,
  any-NotReady without Faulted, Degraded without Faulted/NotReady, HttpStatus
  per-verdict theory.
- LogContextEnricherTests (8): all 4 properties attach; scope disposes cleanly;
  NewCorrelationId shape; null/whitespace driverInstanceId throws.
- CapabilityInvokerEnrichmentTests (2): inner logs carry structured
  properties; no context leak outside the call site.

Full solution dotnet test: 1016 passing (baseline 906, +110 for Phase 6.1 so
far across Streams A+B+C). Pre-existing Client.CLI Subscribe flake unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Core/Resilience/CapabilityInvoker.cs

@@ -1,5 +1,6 @@
 using Polly;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.Observability;
 
 namespace ZB.MOM.WW.OtOpcUa.Core.Resilience;
 
@@ -19,6 +20,7 @@ public sealed class CapabilityInvoker
 {
     private readonly DriverResiliencePipelineBuilder _builder;
     private readonly string _driverInstanceId;
+    private readonly string _driverType;
     private readonly Func<DriverResilienceOptions> _optionsAccessor;
 
     /// <summary>
@@ -30,16 +32,19 @@ public sealed class CapabilityInvoker
     ///     Snapshot accessor for the current resilience options. Invoked per call so Admin-edit +
     ///     pipeline-invalidate can take effect without restarting the invoker.
     /// </param>
+    /// <param name="driverType">Driver type name for structured-log enrichment (e.g. <c>"Modbus"</c>).</param>
     public CapabilityInvoker(
         DriverResiliencePipelineBuilder builder,
         string driverInstanceId,
-        Func<DriverResilienceOptions> optionsAccessor)
+        Func<DriverResilienceOptions> optionsAccessor,
+        string driverType = "Unknown")
     {
         ArgumentNullException.ThrowIfNull(builder);
         ArgumentNullException.ThrowIfNull(optionsAccessor);
 
         _builder = builder;
         _driverInstanceId = driverInstanceId;
+        _driverType = driverType;
         _optionsAccessor = optionsAccessor;
     }
 
@@ -54,7 +59,10 @@ public sealed class CapabilityInvoker
         ArgumentNullException.ThrowIfNull(callSite);
 
         var pipeline = ResolvePipeline(capability, hostName);
-        return await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+        using (LogContextEnricher.Push(_driverInstanceId, _driverType, capability, LogContextEnricher.NewCorrelationId()))
+        {
+            return await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+        }
     }
 
     /// <summary>Execute a void-returning capability call, honoring the per-capability pipeline.</summary>
@@ -67,7 +75,10 @@ public sealed class CapabilityInvoker
         ArgumentNullException.ThrowIfNull(callSite);
 
         var pipeline = ResolvePipeline(capability, hostName);
-        await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+        using (LogContextEnricher.Push(_driverInstanceId, _driverType, capability, LogContextEnricher.NewCorrelationId()))
+        {
+            await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+        }
     }
 
     /// <summary>
@@ -95,7 +106,10 @@ public sealed class CapabilityInvoker
                 },
             };
             var pipeline = _builder.GetOrCreate(_driverInstanceId, $"{hostName}::non-idempotent", DriverCapability.Write, noRetryOptions);
-            return await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+            using (LogContextEnricher.Push(_driverInstanceId, _driverType, DriverCapability.Write, LogContextEnricher.NewCorrelationId()))
+            {
+                return await pipeline.ExecuteAsync(callSite, cancellationToken).ConfigureAwait(false);
+            }
         }
 
         return await ExecuteAsync(DriverCapability.Write, hostName, callSite, cancellationToken).ConfigureAwait(false);