Compositor: read raw keyboard records from stdin and dispatch them into
the real host_kbd_key/host_kbd_modifiers handlers, so keybindings (F11,
Ctrl+PageUp/Down, F9/F10 dumps) can be driven without a focused host
window or wtype. Binary wire: 1|u32 key|u32 state, 2|u32 mods|...
Tests: tests/wltest.c solid-colour xdg_toplevel client, plus a pytest
suite (tests/test_smoke.py) that launches nested instances per test and
feeds events through the new channel. test.sh runs it. F11 fullscreen
toggle now logs its new state.
