Fix second-pass review findings: subscription leak on rebuild, metrics accuracy, and MxAccess startup recovery

- Preserve and replay subscription ref counts across address space rebuilds to prevent MXAccess subscription leaks
- Mark read timeouts and write failures as unsuccessful in PerformanceMetrics for accurate health reporting
- Add deferred MxAccess reconnect path when initial connection fails at startup
- Update code review document with verified completions and new findings
- Add covering tests for all fixes

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

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

@@ -145,15 +145,21 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
             lock (Lock)
             {
                 Log.Information("Rebuilding address space...");
+                var activeSubscriptions = new Dictionary<string, int>(_subscriptionRefCounts, StringComparer.OrdinalIgnoreCase);
 
-                // Remove all predefined nodes
-                var nodesToRemove = new List<NodeId>();
-                foreach (var kvp in _nodeIdToTagReference)
+                foreach (var tagRef in activeSubscriptions.Keys)
                 {
-                    var nodeId = new NodeId(kvp.Key, NamespaceIndex);
-                    nodesToRemove.Add(nodeId);
+                    try
+                    {
+                        _mxAccessClient.UnsubscribeAsync(tagRef).GetAwaiter().GetResult();
+                    }
+                    catch (Exception ex)
+                    {
+                        Log.Warning(ex, "Failed to unsubscribe {TagRef} during rebuild", tagRef);
+                    }
                 }
 
+                // Remove all predefined nodes
                 foreach (var nodeId in PredefinedNodes.Keys.ToList())
                 {
                     try { DeleteNode(SystemContext, nodeId); }
@@ -167,6 +173,23 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
 
                 // Rebuild
                 BuildAddressSpace(hierarchy, attributes);
+
+                foreach (var kvp in activeSubscriptions)
+                {
+                    if (!_tagToVariableNode.ContainsKey(kvp.Key))
+                        continue;
+
+                    try
+                    {
+                        _mxAccessClient.SubscribeAsync(kvp.Key, (_, _) => { }).GetAwaiter().GetResult();
+                        _subscriptionRefCounts[kvp.Key] = kvp.Value;
+                    }
+                    catch (Exception ex)
+                    {
+                        Log.Warning(ex, "Failed to restore subscription for {TagRef} after rebuild", kvp.Key);
+                    }
+                }
+
                 Log.Information("Address space rebuild complete");
             }
         }