feat: establish legacy parity migration baseline

src/MBN_STOCK_WEBVIEW.LegacyParityApp/App.xaml.cs

@@ -0,0 +1,48 @@
+using Microsoft.Windows.AppLifecycle;
+
+namespace MBN_STOCK_WEBVIEW.LegacyParityApp;
+
+public partial class App : Application
+{
+    private const string MainInstanceKey = "Wickedness.MBNStockWebView.LegacyParity.Main";
+
+    private Window? _window;
+    private AppInstance? _mainInstance;
+
+    public App()
+    {
+        InitializeComponent();
+    }
+
+    protected override async void OnLaunched(LaunchActivatedEventArgs args)
+    {
+        var registeredInstance = AppInstance.FindOrRegisterForKey(MainInstanceKey);
+        if (!registeredInstance.IsCurrent)
+        {
+            try
+            {
+                var activation = AppInstance.GetCurrent().GetActivatedEventArgs();
+                if (activation is not null)
+                {
+                    await registeredInstance.RedirectActivationToAsync(activation);
+                }
+            }
+            finally
+            {
+                Exit();
+            }
+
+            return;
+        }
+
+        _mainInstance = registeredInstance;
+        _mainInstance.Activated += OnMainInstanceActivated;
+        _window = new MainWindow();
+        _window.Activate();
+    }
+
+    private void OnMainInstanceActivated(object? sender, AppActivationArguments args)
+    {
+        _window?.DispatcherQueue.TryEnqueue(_window.Activate);
+    }
+}