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.Server/OpcUa/OpcUaApplicationHost.cs

@@ -4,6 +4,7 @@ using Opc.Ua.Configuration;
 using ZB.MOM.WW.OtOpcUa.Core.Hosting;
 using ZB.MOM.WW.OtOpcUa.Core.OpcUa;
 using ZB.MOM.WW.OtOpcUa.Core.Resilience;
+using ZB.MOM.WW.OtOpcUa.Server.Observability;
 using ZB.MOM.WW.OtOpcUa.Server.Security;
 
 namespace ZB.MOM.WW.OtOpcUa.Server.OpcUa;
@@ -26,6 +27,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
     private readonly ILogger<OpcUaApplicationHost> _logger;
     private ApplicationInstance? _application;
     private OtOpcUaServer? _server;
+    private HealthEndpointsHost? _healthHost;
     private bool _disposed;
 
     public OpcUaApplicationHost(OpcUaServerOptions options, DriverHost driverHost,
@@ -68,6 +70,17 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         _logger.LogInformation("OPC UA server started — endpoint={Endpoint} driverCount={Count}",
             _options.EndpointUrl, _server.DriverNodeManagers.Count);
 
+        // Phase 6.1 Stream C: health endpoints on :4841 (loopback by default — see
+        // HealthEndpointsHost remarks for the Windows URL-ACL tradeoff).
+        if (_options.HealthEndpointsEnabled)
+        {
+            _healthHost = new HealthEndpointsHost(
+                _driverHost,
+                _loggerFactory.CreateLogger<HealthEndpointsHost>(),
+                prefix: _options.HealthEndpointsPrefix);
+            _healthHost.Start();
+        }
+
         // Drive each driver's discovery through its node manager. The node manager IS the
         // IAddressSpaceBuilder; GenericDriverNodeManager captures alarm-condition sinks into
         // its internal map and wires OnAlarmEvent → sink routing.
@@ -221,6 +234,12 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         {
             _logger.LogWarning(ex, "OPC UA server stop threw during dispose");
         }
+
+        if (_healthHost is not null)
+        {
+            try { await _healthHost.DisposeAsync().ConfigureAwait(false); }
+            catch (Exception ex) { _logger.LogWarning(ex, "Health endpoints host dispose threw"); }
+        }
         await Task.CompletedTask;
     }
 }