Input and windows

‍Draft: API contracts and SDL integration examples are still needed.

Window lifecycle

TODO: Explain creating a window, resizing, DPI/high-density handling, and destruction.

Event handling

TODO: Document how input and window events travel through Flint, including focus and quit handling.

Keyboard, mouse, and controllers

TODO: List supported input devices, bindings, and platform limitations.

Coordinate conversion

TODO: Show input coordinates relative to viewport and camera when appropriate.

See Architecture.