Initial commit: .NET MAUI Linux Platform

Complete Linux platform implementation for .NET MAUI with:

- 35+ Skia-rendered controls (Button, Label, Entry, CarouselView, etc.)
- Platform services (Clipboard, FilePicker, Notifications, DragDrop, etc.)
- Accessibility support (AT-SPI2, High Contrast)
- HiDPI and Input Method support
- 216 unit tests
- CI/CD workflows
- Project templates
- Documentation

🤖 Generated with Claude Code

Input/Key.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.Maui.Platform;
+
+/// <summary>
+/// Keyboard key enumeration.
+/// </summary>
+public enum Key
+{
+    Unknown = 0,
+
+    // Letters
+    A, B, C, D, E, F, G, H, I, J, K, L, M,
+    N, O, P, Q, R, S, T, U, V, W, X, Y, Z,
+
+    // Numbers
+    D0, D1, D2, D3, D4, D5, D6, D7, D8, D9,
+
+    // Numpad
+    NumPad0, NumPad1, NumPad2, NumPad3, NumPad4,
+    NumPad5, NumPad6, NumPad7, NumPad8, NumPad9,
+    NumPadMultiply, NumPadAdd, NumPadSubtract,
+    NumPadDecimal, NumPadDivide, NumPadEnter,
+
+    // Function keys
+    F1, F2, F3, F4, F5, F6, F7, F8, F9, F10, F11, F12,
+
+    // Navigation
+    Left, Up, Right, Down,
+    Home, End, PageUp, PageDown,
+    Insert, Delete,
+
+    // Modifiers
+    Shift, Control, Alt, Super,
+    CapsLock, NumLock, ScrollLock,
+
+    // Editing
+    Backspace, Tab, Enter, Escape, Space,
+
+    // Punctuation
+    Comma, Period, Slash, Semicolon, Quote,
+    LeftBracket, RightBracket, Backslash,
+    Minus, Equals, Grave,
+
+    // System
+    PrintScreen, Pause, Menu,
+}