feat(runtime): publish DriverHealthChanged via DriverInstanceActor

- IDriverHealthPublisher in Core.Abstractions + NullDriverHealthPublisher
  no-op for tests/dev-stub paths.
- AkkaDriverHealthPublisher in Runtime forwards to the cluster-wide
  `driver-health` DPS topic.
- DriverInstanceActor instrumented to publish snapshots on every
  observable state change + a periodic 30s heartbeat so the AdminUI
  snapshot store warms up for newly-joined SignalR clients.
- Sliding 5-minute Faulted-count tracked per actor via Queue<DateTime>.
- DriverHostActor.SpawnChild threads clusterId (_localNode.Value) and
  the health publisher down to every DriverInstanceActor child.
- ServiceCollectionExtensions.AddOtOpcUaRuntime registers
  AkkaDriverHealthPublisher as IDriverHealthPublisher singleton.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverHealthPublisher.cs

@@ -0,0 +1,39 @@
+namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+/// <summary>
+///     Sink for driver-health state-change notifications. The runtime DI wires the
+///     Akka-DistributedPubSub-backed implementation; tests and dev-stub paths use
+///     <see cref="NullDriverHealthPublisher"/> to opt out without changing call sites.
+/// </summary>
+public interface IDriverHealthPublisher
+{
+    /// <summary>
+    ///     Publishes a health snapshot for one driver instance. Implementations must be
+    ///     non-blocking and tolerant of being called from any thread.
+    /// </summary>
+    void Publish(
+        string clusterId,
+        string driverInstanceId,
+        DriverHealth health,
+        int errorCount5Min);
+}
+
+/// <summary>
+///     Drop-in no-op for tests and dev-stub paths. Production wires the Akka-backed
+///     implementation in the Runtime project.
+/// </summary>
+public sealed class NullDriverHealthPublisher : IDriverHealthPublisher
+{
+    /// <summary>Singleton instance.</summary>
+    public static readonly NullDriverHealthPublisher Instance = new();
+
+    private NullDriverHealthPublisher() { }
+
+    /// <inheritdoc />
+    public void Publish(
+        string clusterId,
+        string driverInstanceId,
+        DriverHealth health,
+        int errorCount5Min)
+    { /* no-op */ }
+}