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.cs

@@ -0,0 +1,92 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Serilog;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+using ZB.MOM.WW.LmxOpcUa.Host.Domain;
+using ZB.MOM.WW.LmxOpcUa.Host.Metrics;
+
+namespace ZB.MOM.WW.LmxOpcUa.Host.MxAccess
+{
+    /// <summary>
+    /// Core MXAccess client implementing IMxAccessClient via IMxProxy abstraction.
+    /// Split across partial classes: Connection, Subscription, ReadWrite, EventHandlers, Monitor.
+    /// (MXA-001 through MXA-009)
+    /// </summary>
+    public sealed partial class MxAccessClient : IMxAccessClient
+    {
+        private static readonly ILogger Log = Serilog.Log.ForContext<MxAccessClient>();
+
+        private readonly StaComThread _staThread;
+        private readonly IMxProxy _proxy;
+        private readonly MxAccessConfiguration _config;
+        private readonly PerformanceMetrics _metrics;
+        private readonly SemaphoreSlim _operationSemaphore;
+
+        private int _connectionHandle;
+        private volatile ConnectionState _state = ConnectionState.Disconnected;
+        private CancellationTokenSource? _monitorCts;
+
+        // Handle mappings
+        private readonly ConcurrentDictionary<int, string> _handleToAddress = new ConcurrentDictionary<int, string>();
+        private readonly ConcurrentDictionary<string, int> _addressToHandle = new ConcurrentDictionary<string, int>(StringComparer.OrdinalIgnoreCase);
+
+        // Subscription storage
+        private readonly ConcurrentDictionary<string, Action<string, Vtq>> _storedSubscriptions
+            = new ConcurrentDictionary<string, Action<string, Vtq>>(StringComparer.OrdinalIgnoreCase);
+
+        // Pending writes
+        private readonly ConcurrentDictionary<int, TaskCompletionSource<bool>> _pendingWrites
+            = new ConcurrentDictionary<int, TaskCompletionSource<bool>>();
+
+        // Probe
+        private string? _probeTag;
+        private DateTime _lastProbeValueTime = DateTime.UtcNow;
+        private int _reconnectCount;
+
+        public ConnectionState State => _state;
+        public int ActiveSubscriptionCount => _storedSubscriptions.Count;
+        public int ReconnectCount => _reconnectCount;
+
+        public event EventHandler<ConnectionStateChangedEventArgs>? ConnectionStateChanged;
+        public event Action<string, Vtq>? OnTagValueChanged;
+
+        public MxAccessClient(StaComThread staThread, IMxProxy proxy, MxAccessConfiguration config, PerformanceMetrics metrics)
+        {
+            _staThread = staThread;
+            _proxy = proxy;
+            _config = config;
+            _metrics = metrics;
+            _operationSemaphore = new SemaphoreSlim(config.MaxConcurrentOperations, config.MaxConcurrentOperations);
+        }
+
+        private void SetState(ConnectionState newState, string message = "")
+        {
+            var previous = _state;
+            if (previous == newState) return;
+            _state = newState;
+            Log.Information("MxAccess state: {Previous} → {Current} {Message}", previous, newState, message);
+            ConnectionStateChanged?.Invoke(this, new ConnectionStateChangedEventArgs(previous, newState, message));
+        }
+
+        public void Dispose()
+        {
+            try
+            {
+                _monitorCts?.Cancel();
+                DisconnectAsync().GetAwaiter().GetResult();
+            }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "Error during MxAccessClient dispose");
+            }
+            finally
+            {
+                _operationSemaphore.Dispose();
+                _monitorCts?.Dispose();
+            }
+        }
+    }
+}