test: optional live input test (wheel, mouse encodings, prompt jumping)
Runs term in an isolated headless sway and delivers real Wayland events: a
persistent zwlr_virtual_pointer_v1 client (pywayland, bindings generated from
the protocol XML at run time) for wheel and clicks, and wtype for keys. Checks
alt scroll, the four mouse encodings and Ctrl+Shift+Z/X prompt jumps by what
the program inside receives and by screenshot pixels.
Lives in its own dev shell (nix develop .#live-input) and is not part of CI or
the build.