Native windowing backends for Windows and Linux (design + factory seam)

[codymullins]

Problem

Skyline.Apple gives macOS real native chrome (transparent/unified title bar) and is where native IME/a11y/cursor live. Windows and Linux only have the portable GLFW path. Eventually they want native backends for OS-native chrome and the native services GLFW can't reach.

Current state

• src/Skyline/WindowBackendFactory.cs already picks a backend and reflectively loads Skyline.Apple on macOS, falling back to GLFW — the seam is extensible.
• src/Skyline/IWindowBackend.cs is the contract; src/Skyline/AppKitChromeMap.cs shows the chrome-mapping pattern.

What's needed

• A short design doc for Skyline.Windows (Win32) and Skyline.Linux (X11/Wayland) backends: window creation, event pump/threading contract, chrome mapping, surface factory, and the native services (cursor, IME, a11y, clipboard) they'd own.
• Extend the factory to try-load these by name before GLFW, falling back cleanly.
• Optional stub projects to lock the seam.

Acceptance criteria

• Design doc agreed; factory extension (or documented plan) in place. GLFW remains the supported fallback — this is not required to ship cross-platform on GLFW.