Add full XAML support for .NET MAUI compatibility

New features:
- MauiAppBuilderExtensions.UseOpenMauiLinux() for standard MAUI integration
- New template: openmaui-linux-xaml with full XAML support
- Standard MAUI XAML syntax (ContentPage, VerticalStackLayout, etc.)
- Resource dictionaries (Colors.xaml, Styles.xaml)
- Compiled XAML support via MauiXaml items

Templates now available:
- dotnet new openmaui-linux      (code-based UI)
- dotnet new openmaui-linux-xaml (XAML-based UI)

Usage:
    var builder = MauiApp.CreateBuilder();
    builder
        .UseMauiApp<App>()
        .UseOpenMauiLinux();  // Enable Linux with XAML

templates/openmaui-linux-xaml-app/MainPage.xaml.cs

@@ -0,0 +1,30 @@
+namespace OpenMauiXamlApp;
+
+public partial class MainPage : ContentPage
+{
+    private int _count = 0;
+
+    public MainPage()
+    {
+        InitializeComponent();
+
+        // Wire up slider value changed
+        VolumeSlider.ValueChanged += OnVolumeChanged;
+    }
+
+    private void OnCounterClicked(object sender, EventArgs e)
+    {
+        _count++;
+
+        CounterBtn.Text = _count == 1
+            ? $"Clicked {_count} time"
+            : $"Clicked {_count} times";
+
+        SemanticScreenReader.Announce(CounterBtn.Text);
+    }
+
+    private void OnVolumeChanged(object? sender, ValueChangedEventArgs e)
+    {
+        VolumeLabel.Text = $"Volume: {e.NewValue:F0}";
+    }
+}