Implement LmxOpcUa server — all 6 phases complete

Full OPC UA server on .NET Framework 4.8 (x86) exposing AVEVA System
Platform Galaxy tags via MXAccess. Mirrors Galaxy object hierarchy as
OPC UA address space, translating contained-name browse paths to
tag-name runtime references.

Components implemented:
- Configuration: AppConfiguration with 4 sections, validator
- Domain: ConnectionState, Quality, Vtq, MxDataTypeMapper, error codes
- MxAccess: StaComThread, MxAccessClient (partial classes), MxProxyAdapter
  using strongly-typed ArchestrA.MxAccess COM interop
- Galaxy Repository: SQL queries (hierarchy, attributes, change detection),
  ChangeDetectionService with auto-rebuild on deploy
- OPC UA Server: LmxNodeManager (CustomNodeManager2), LmxOpcUaServer,
  OpcUaServerHost with programmatic config, SecurityPolicy None
- Status Dashboard: HTTP server with HTML/JSON/health endpoints
- Integration: Full 14-step startup, graceful shutdown, component wiring

175 tests (174 unit + 1 integration), all passing.

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

src/ZB.MOM.WW.LmxOpcUa.Host/MxAccess/MxAccessClient.Subscription.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Threading.Tasks;
+using Serilog;
+using ZB.MOM.WW.LmxOpcUa.Host.Domain;
+
+namespace ZB.MOM.WW.LmxOpcUa.Host.MxAccess
+{
+    public sealed partial class MxAccessClient
+    {
+        public async Task SubscribeAsync(string fullTagReference, Action<string, Vtq> callback)
+        {
+            _storedSubscriptions[fullTagReference] = callback;
+            if (_state != ConnectionState.Connected) return;
+
+            await SubscribeInternalAsync(fullTagReference);
+        }
+
+        public async Task UnsubscribeAsync(string fullTagReference)
+        {
+            _storedSubscriptions.TryRemove(fullTagReference, out _);
+
+            // Don't unsubscribe the probe tag
+            if (string.Equals(fullTagReference, _probeTag, StringComparison.OrdinalIgnoreCase))
+                return;
+
+            if (_addressToHandle.TryRemove(fullTagReference, out var itemHandle))
+            {
+                _handleToAddress.TryRemove(itemHandle, out _);
+
+                if (_state == ConnectionState.Connected)
+                {
+                    await _staThread.RunAsync(() =>
+                    {
+                        try
+                        {
+                            _proxy.UnAdviseSupervisory(_connectionHandle, itemHandle);
+                            _proxy.RemoveItem(_connectionHandle, itemHandle);
+                        }
+                        catch (Exception ex)
+                        {
+                            Log.Warning(ex, "Error unsubscribing {Address}", fullTagReference);
+                        }
+                    });
+                }
+            }
+        }
+
+        private async Task SubscribeInternalAsync(string address)
+        {
+            using var scope = _metrics.BeginOperation("Subscribe");
+            try
+            {
+                var itemHandle = await _staThread.RunAsync(() =>
+                {
+                    var h = _proxy.AddItem(_connectionHandle, address);
+                    _proxy.AdviseSupervisory(_connectionHandle, h);
+                    return h;
+                });
+
+                _handleToAddress[itemHandle] = address;
+                _addressToHandle[address] = itemHandle;
+                Log.Debug("Subscribed to {Address} (handle={Handle})", address, itemHandle);
+            }
+            catch (Exception ex)
+            {
+                scope.SetSuccess(false);
+                Log.Error(ex, "Failed to subscribe to {Address}", address);
+                throw;
+            }
+        }
+
+        private async Task ReplayStoredSubscriptionsAsync()
+        {
+            foreach (var kvp in _storedSubscriptions)
+            {
+                try
+                {
+                    await SubscribeInternalAsync(kvp.Key);
+                }
+                catch (Exception ex)
+                {
+                    Log.Warning(ex, "Failed to replay subscription for {Address}", kvp.Key);
+                }
+            }
+
+            Log.Information("Replayed {Count} stored subscriptions", _storedSubscriptions.Count);
+        }
+    }
+}