Add cross-platform OPC UA client stack: shared library, CLI tool, and Avalonia UI

Implements Client.Shared (IOpcUaClientService with connection lifecycle, failover,
browse, read/write, subscriptions, alarms, history, redundancy), Client.CLI (8 CliFx
commands mirroring tools/opcuacli-dotnet), and Client.UI (Avalonia desktop app with
tree browser, read/write, subscriptions, alarms, and history tabs). All three target
.NET 10 and are covered by 249 unit tests.

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

src/ZB.MOM.WW.LmxOpcUa.Client.UI/Services/AvaloniaUiDispatcher.cs

@@ -0,0 +1,14 @@
+using Avalonia.Threading;
+
+namespace ZB.MOM.WW.LmxOpcUa.Client.UI.Services;
+
+/// <summary>
+/// Dispatches actions to the Avalonia UI thread.
+/// </summary>
+public sealed class AvaloniaUiDispatcher : IUiDispatcher
+{
+    public void Post(Action action)
+    {
+        Dispatcher.UIThread.Post(action);
+    }
+}

src/ZB.MOM.WW.LmxOpcUa.Client.UI/Services/IUiDispatcher.cs

@@ -0,0 +1,12 @@
+namespace ZB.MOM.WW.LmxOpcUa.Client.UI.Services;
+
+/// <summary>
+/// Abstraction for dispatching actions to the UI thread.
+/// </summary>
+public interface IUiDispatcher
+{
+    /// <summary>
+    /// Posts an action to be executed on the UI thread.
+    /// </summary>
+    void Post(Action action);
+}

src/ZB.MOM.WW.LmxOpcUa.Client.UI/Services/SynchronousUiDispatcher.cs

@@ -0,0 +1,13 @@
+namespace ZB.MOM.WW.LmxOpcUa.Client.UI.Services;
+
+/// <summary>
+/// Dispatcher that executes actions synchronously on the calling thread.
+/// Used for unit testing where no UI thread is available.
+/// </summary>
+public sealed class SynchronousUiDispatcher : IUiDispatcher
+{
+    public void Post(Action action)
+    {
+        action();
+    }
+}