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>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Integration/AddressSpaceRebuildTests.cs

@@ -259,5 +259,32 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Integration
                 await fixture.DisposeAsync();
             }
         }
+
+        [Fact]
+        public async Task Rebuild_PreservesSubscriptionBookkeeping_ForSurvivingNodes()
+        {
+            var fixture = OpcUaServerFixture.WithFakeMxAccessClient();
+            await fixture.InitializeAsync();
+            try
+            {
+                var nodeManager = fixture.Service.NodeManagerInstance!;
+                var mxClient = fixture.MxAccessClient!;
+
+                nodeManager.SubscribeTag("TestMachine_001.MachineID");
+                mxClient.ActiveSubscriptionCount.ShouldBe(1);
+
+                fixture.Service.TriggerRebuild();
+                await Task.Delay(200);
+
+                mxClient.ActiveSubscriptionCount.ShouldBe(1);
+
+                nodeManager.UnsubscribeTag("TestMachine_001.MachineID");
+                mxClient.ActiveSubscriptionCount.ShouldBe(0);
+            }
+            finally
+            {
+                await fixture.DisposeAsync();
+            }
+        }
     }
 }