Single-file Wayland compositor presenting client surfaces as tabs in a
GTK3 window. Implements wl_compositor, wl_shm/shared memory, xdg-shell,
wl_seat/keyboard/pointer, and launches apps from tab-commands.
- Validate client buffer geometry against pool size at create_buffer and
commit time to prevent out-of-bounds reads from malformed buffers.
- Release client shm buffers immediately after deep-copying pixels,
replacing the fragile foot/GTK3 deferred-release state machine.
