Add control gallery sample and roadmap documentation

- Add comprehensive ControlGallery sample app with 12 pages
  demonstrating all 35+ controls
- Add detailed ROADMAP.md with version milestones
- Add README placeholders for VSIX icons and template images
- Sample pages include: Home, Buttons, Labels, Entry, Pickers,
  Sliders, Toggles, Progress, Images, CollectionView, CarouselView,
  SwipeView, RefreshView

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

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

samples/ControlGallery/Pages/CollectionViewPage.xaml.cs

@@ -0,0 +1,40 @@
+namespace ControlGallery.Pages;
+
+public partial class CollectionViewPage : ContentPage
+{
+    public CollectionViewPage()
+    {
+        InitializeComponent();
+
+        var items = new List<ItemModel>
+        {
+            new() { Title = "Product A", Description = "High quality item", Price = 29.99m, Color = Colors.Purple },
+            new() { Title = "Product B", Description = "Best seller", Price = 49.99m, Color = Colors.Blue },
+            new() { Title = "Product C", Description = "New arrival", Price = 19.99m, Color = Colors.Green },
+            new() { Title = "Product D", Description = "Limited edition", Price = 99.99m, Color = Colors.Orange },
+            new() { Title = "Product E", Description = "Customer favorite", Price = 39.99m, Color = Colors.Red },
+            new() { Title = "Product F", Description = "Eco-friendly", Price = 24.99m, Color = Colors.Teal },
+            new() { Title = "Product G", Description = "Premium quality", Price = 79.99m, Color = Colors.Indigo },
+            new() { Title = "Product H", Description = "Budget friendly", Price = 9.99m, Color = Colors.Pink },
+        };
+
+        ItemsCollection.ItemsSource = items;
+    }
+
+    private async void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
+    {
+        if (e.CurrentSelection.FirstOrDefault() is ItemModel item)
+        {
+            await DisplayAlert("Selected", $"You selected {item.Title}", "OK");
+            ((CollectionView)sender).SelectedItem = null;
+        }
+    }
+}
+
+public class ItemModel
+{
+    public string Title { get; set; } = string.Empty;
+    public string Description { get; set; } = string.Empty;
+    public decimal Price { get; set; }
+    public Color Color { get; set; } = Colors.Gray;
+}