Instrument the historian plugin with runtime query health counters and read-only cluster failover so operators can detect silent query degradation and keep serving history when a single cluster node goes down

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.LmxOpcUa.Host/OpcUa/LmxNodeManager.cs

@@ -190,6 +190,13 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
         public IReadOnlyList<string> AlarmFilterPatterns =>
             _alarmObjectFilter?.RawPatterns ?? Array.Empty<string>();
 
+        /// <summary>
+        ///     Gets the runtime historian health snapshot, or <see langword="null"/> when the historian
+        ///     plugin is not loaded. Surfaced on the status dashboard so operators can detect query
+        ///     failures that the load-time plugin status cannot catch.
+        /// </summary>
+        public HistorianHealthSnapshot? HistorianHealth => _historianDataSource?.GetHealthSnapshot();
+
         /// <summary>
         ///     Gets the number of distinct alarm conditions currently tracked (one per alarm attribute).
         /// </summary>