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.Shared/Models/SecurityMode.cs

@@ -0,0 +1,16 @@
+namespace ZB.MOM.WW.LmxOpcUa.Client.Shared.Models;
+
+/// <summary>
+/// Transport security mode for the OPC UA connection.
+/// </summary>
+public enum SecurityMode
+{
+    /// <summary>No transport security.</summary>
+    None,
+
+    /// <summary>Messages are signed but not encrypted.</summary>
+    Sign,
+
+    /// <summary>Messages are signed and encrypted.</summary>
+    SignAndEncrypt
+}