Add ShellDemo sample with comprehensive XAML controls showcase

Complete ShellDemo application demonstrating all MAUI controls:
- App/AppShell: Shell navigation with flyout menu
- HomePage: Feature cards, theme toggle, quick actions
- ButtonsPage: Button styles, states, variations, event logging
- TextInputPage: Entry, Editor, SearchBar with keyboard shortcuts
- SelectionPage: CheckBox, Switch, Slider with colored variants
- PickersPage: Picker, DatePicker, TimePicker demos
- ListsPage: CollectionView with fruits, colors, contacts
- ProgressPage: ProgressBar, ActivityIndicator, interactive demo
- GridsPage: Grid layouts - auto/star/absolute sizing, spans, nesting
- AboutPage: OpenMaui Linux information
- DetailPage: Push/pop navigation demo

All pages use proper XAML with code-behind following MAUI patterns.

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

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

samples/ShellDemo/Pages/SelectionPage.xaml.cs

@@ -0,0 +1,70 @@
+using System;
+using Microsoft.Maui.Controls;
+
+namespace ShellDemo.Pages;
+
+public partial class SelectionPage : ContentPage
+{
+    private int _eventCount = 0;
+
+    public SelectionPage()
+    {
+        InitializeComponent();
+    }
+
+    private void LogEvent(string message)
+    {
+        _eventCount++;
+        var timestamp = DateTime.Now.ToString("HH:mm:ss");
+        EventLog.Text = $"[{timestamp}] {_eventCount}. {message}\n{EventLog.Text}";
+    }
+
+    private void OnCheckbox1Changed(object? sender, CheckedChangedEventArgs e)
+    {
+        LogEvent($"Checkbox 1: {(e.Value ? "Checked" : "Unchecked")}");
+    }
+
+    private void OnCheckbox2Changed(object? sender, CheckedChangedEventArgs e)
+    {
+        LogEvent($"Checkbox 2: {(e.Value ? "Checked" : "Unchecked")}");
+    }
+
+    private void OnColoredCheckboxChanged(object? sender, CheckedChangedEventArgs e)
+    {
+        if (sender is CheckBox cb)
+        {
+            LogEvent($"{cb.Color} checkbox: {(e.Value ? "Checked" : "Unchecked")}");
+        }
+    }
+
+    private void OnBasicSwitchToggled(object? sender, ToggledEventArgs e)
+    {
+        SwitchStatusLabel.Text = e.Value ? "On" : "Off";
+        LogEvent($"Switch toggled: {(e.Value ? "ON" : "OFF")}");
+    }
+
+    private void OnColoredSwitchToggled(object? sender, ToggledEventArgs e)
+    {
+        if (sender is Switch sw)
+        {
+            LogEvent($"{sw.OnColor} switch: {(e.Value ? "ON" : "OFF")}");
+        }
+    }
+
+    private void OnBasicSliderChanged(object? sender, ValueChangedEventArgs e)
+    {
+        SliderValueLabel.Text = $"Value: {(int)e.NewValue}";
+        LogEvent($"Slider value: {(int)e.NewValue}");
+    }
+
+    private void OnTempSliderChanged(object? sender, ValueChangedEventArgs e)
+    {
+        TempLabel.Text = $"{(int)e.NewValue}C";
+        LogEvent($"Temperature: {(int)e.NewValue}C");
+    }
+
+    private void OnColoredSliderChanged(object? sender, ValueChangedEventArgs e)
+    {
+        LogEvent($"Colored slider: {(int)e.NewValue}");
+    }
+}