Preview 3: Complete control implementation with XAML data binding

Major milestone adding full control functionality:

Controls Enhanced:
- Entry/Editor: Full keyboard input, cursor navigation, selection, clipboard
- CollectionView: Data binding, selection highlighting, scrolling
- CheckBox/Switch/Slider: Interactive state management
- Picker/DatePicker/TimePicker: Dropdown selection with popup overlays
- ProgressBar/ActivityIndicator: Animated progress display
- Button: Press/release visual states
- Border/Frame: Rounded corners, stroke styling
- Label: Text wrapping, alignment, decorations
- Grid/StackLayout: Margin and padding support

Features Added:
- DisplayAlert dialogs with button actions
- NavigationPage with toolbar and back navigation
- Shell with flyout menu navigation
- XAML value converters for data binding
- Margin support in all layout containers
- Popup overlay system for pickers

New Samples:
- TodoApp: Full CRUD task manager with NavigationPage
- ShellDemo: Comprehensive control showcase

Removed:
- ControlGallery (replaced by ShellDemo)
- LinuxDemo (replaced by TodoApp)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Views/SkiaBoxView.cs

@@ -0,0 +1,66 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using SkiaSharp;
+
+namespace Microsoft.Maui.Platform;
+
+/// <summary>
+/// Skia-rendered BoxView - a simple colored rectangle.
+/// </summary>
+public class SkiaBoxView : SkiaView
+{
+    public static readonly BindableProperty ColorProperty =
+        BindableProperty.Create(nameof(Color), typeof(SKColor), typeof(SkiaBoxView), SKColors.Transparent,
+            propertyChanged: (b, o, n) => ((SkiaBoxView)b).Invalidate());
+
+    public static readonly BindableProperty CornerRadiusProperty =
+        BindableProperty.Create(nameof(CornerRadius), typeof(float), typeof(SkiaBoxView), 0f,
+            propertyChanged: (b, o, n) => ((SkiaBoxView)b).Invalidate());
+
+    public SKColor Color
+    {
+        get => (SKColor)GetValue(ColorProperty);
+        set => SetValue(ColorProperty, value);
+    }
+
+    public float CornerRadius
+    {
+        get => (float)GetValue(CornerRadiusProperty);
+        set => SetValue(CornerRadiusProperty, value);
+    }
+
+    protected override void OnDraw(SKCanvas canvas, SKRect bounds)
+    {
+        using var paint = new SKPaint
+        {
+            Color = Color,
+            Style = SKPaintStyle.Fill,
+            IsAntialias = true
+        };
+
+        if (CornerRadius > 0)
+        {
+            canvas.DrawRoundRect(bounds, CornerRadius, CornerRadius, paint);
+        }
+        else
+        {
+            canvas.DrawRect(bounds, paint);
+        }
+    }
+
+    protected override SKSize MeasureOverride(SKSize availableSize)
+    {
+        // BoxView uses explicit size or a default size when in unbounded context
+        var width = WidthRequest >= 0 ? (float)WidthRequest :
+                    (float.IsInfinity(availableSize.Width) ? 40f : availableSize.Width);
+        var height = HeightRequest >= 0 ? (float)HeightRequest :
+                     (float.IsInfinity(availableSize.Height) ? 40f : availableSize.Height);
+
+        // Ensure no NaN values
+        if (float.IsNaN(width)) width = 40f;
+        if (float.IsNaN(height)) height = 40f;
+
+        return new SKSize(width, height);
+    }
+}