feat(commons): add cluster/admin/diagnostics client interfaces

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Interfaces/IFleetDiagnosticsClient.cs

@@ -0,0 +1,12 @@
+using ZB.MOM.WW.OtOpcUa.Commons.Types;
+
+namespace ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
+
+/// <summary>
+/// Per-node diagnostics fetched on demand. Implemented in Phase 8 (AdminUI/Runtime wiring)
+/// over an Akka request/response — the diagnostics actor lives on the target driver node.
+/// </summary>
+public interface IFleetDiagnosticsClient
+{
+    Task<NodeDiagnosticsSnapshot> GetDiagnosticsAsync(NodeId nodeId, CancellationToken ct);
+}