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).

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/DeferredServiceLevelPublisherTests.cs

@@ -0,0 +1,48 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
+
+public sealed class DeferredServiceLevelPublisherTests
+{
+    [Fact]
+    public void Publish_before_SetInner_is_a_safe_noop()
+    {
+        var deferred = new DeferredServiceLevelPublisher();
+
+        Should.NotThrow(() => deferred.Publish(123));
+    }
+
+    [Fact]
+    public void Publish_after_SetInner_routes_to_the_inner()
+    {
+        var recording = new RecordingPublisher();
+        var deferred = new DeferredServiceLevelPublisher();
+        deferred.SetInner(recording);
+
+        deferred.Publish(200);
+
+        recording.LastValue.ShouldBe((byte)200);
+    }
+
+    [Fact]
+    public void SetInner_null_reverts_to_Null_publisher()
+    {
+        var recording = new RecordingPublisher();
+        var deferred = new DeferredServiceLevelPublisher();
+        deferred.SetInner(recording);
+        deferred.Publish(50);
+
+        deferred.SetInner(null);
+        deferred.Publish(99);
+
+        recording.LastValue.ShouldBe((byte)50, "writes after SetInner(null) must not reach the previous inner");
+    }
+
+    private sealed class RecordingPublisher : IServiceLevelPublisher
+    {
+        public byte? LastValue { get; private set; }
+        public void Publish(byte serviceLevel) => LastValue = serviceLevel;
+    }
+}