feat(opcua,host): #81 ServiceLevel SDK publisher

SdkServiceLevelPublisher writes Server.ServiceLevel through the SDK's
ServerObjectState — the standard OPC UA non-transparent-redundancy signal
clients use to pick a primary. Writes are guarded by DiagnosticsLock so
concurrent SDK diagnostics scans don't fight with our updates.

DeferredServiceLevelPublisher mirrors the DeferredAddressSpaceSink late-
binding pattern: Akka actors resolve IServiceLevelPublisher at construction,
hosted service swaps the SDK publisher in after StandardServer.Start. Host
Program.cs registers DeferredServiceLevelPublisher as the singleton bound
to IServiceLevelPublisher; OtOpcUaServerHostedService gets it injected and
fills it once IServerInternal is available.

Tests boot a real StandardServer on a free port (cross-platform), call
Publish, then verify ServerObject.ServiceLevel.Value reflects the write.
5 new tests; OpcUaServer suite now 45/45 green (was 40, +5).

Closes #81 residual. Unblocks Task 60 (OPC UA dual-endpoint + ServiceLevel
tests).

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/OtOpcUaServerHostedService.cs

@@ -22,6 +22,7 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
 {
     private readonly IConfiguration _configuration;
     private readonly DeferredAddressSpaceSink _deferredSink;
+    private readonly DeferredServiceLevelPublisher _deferredServiceLevel;
     private readonly IOpcUaUserAuthenticator _userAuthenticator;
     private readonly ILoggerFactory _loggerFactory;
     private readonly ILogger<OtOpcUaServerHostedService> _logger;
@@ -32,11 +33,13 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
     public OtOpcUaServerHostedService(
         IConfiguration configuration,
         DeferredAddressSpaceSink deferredSink,
+        DeferredServiceLevelPublisher deferredServiceLevel,
         IOpcUaUserAuthenticator userAuthenticator,
         ILoggerFactory loggerFactory)
     {
         _configuration = configuration;
         _deferredSink = deferredSink;
+        _deferredServiceLevel = deferredServiceLevel;
         _userAuthenticator = userAuthenticator;
         _loggerFactory = loggerFactory;
         _logger = loggerFactory.CreateLogger<OtOpcUaServerHostedService>();
@@ -75,14 +78,24 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
         }
 
         _deferredSink.SetSink(new SdkAddressSpaceSink(_server.NodeManager));
-        _logger.LogInformation("OtOpcUaServerHostedService: SDK started, address-space sink bound");
+
+        // ServiceLevel publisher needs IServerInternal — only available after Start.
+        if (_server.CurrentInstance is { } serverInternal)
+        {
+            _deferredServiceLevel.SetInner(new SdkServiceLevelPublisher(
+                serverInternal,
+                _loggerFactory.CreateLogger<SdkServiceLevelPublisher>()));
+        }
+
+        _logger.LogInformation("OtOpcUaServerHostedService: SDK started, address-space + ServiceLevel sinks bound");
     }
 
     public Task StopAsync(CancellationToken cancellationToken)
     {
-        // Revert to Null sink so any in-flight writes from a poison-pilled actor don't hit a
+        // Revert to Null adapters so any in-flight writes from a poison-pilled actor don't hit a
         // half-disposed NodeManager.
         _deferredSink.SetSink(null);
+        _deferredServiceLevel.SetInner(null);
         return Task.CompletedTask;
     }