spike(kernel): RML compositing live + interactive on the real DRM seat

Iterated the Phase-0 spike from "self-verified headless" to "works on the
CF-AX3": a live, 3D-tilted client window you can type into, with a visible
cursor and pointer/touch routed through the transform. Real-seat confirmed.

Safety / escape (a black-screen lockout had forced reboots):
- session escape hatch on --run (mirrors src/input.cpp): Esc + Ctrl+Alt+Backspace
  quit, Ctrl+Alt+F1..F12 VT-switch, SIGINT/SIGTERM clean-terminate.
- 15s DEAD-MAN auto-exit, reset by pressing P (also a keyboard-liveness probe);
  UNBOX_SPIKE_TIMEOUT overrides. Guarantees the spike can never lock the machine.
- persistent crash-survivable log at $HOME/rml-spike.log (UNBOX_SPIKE_LOG),
  fflush+fsync per line (was on tmpfs /tmp -> lost on reboot); loud diagnostics
  for backend/modeset/client-connect/map/import.

Black screen / no window fixes:
- present path works on real DRM+gles2 (blue bg + marker confirmed on panel);
  dark-blue clear + marker rect make "empty vs not-presenting" diagnosable.
- xdg wired from new_toplevel/new_popup (was new_surface -> no role -> no
  configure -> client never mapped); foot now maps as a live surface element.
- WAYLAND_DISPLAY exported so spawned clients connect.

Interactivity:
- keyboard focus enter + key forward to the focused client (was never sent
  without a pre-existing keyboard device); typing reaches foot.
- visible wlr-plane cursor (xcursor theme load + set; never drawn into RmlUi).
- pointer + touch hit-tested via RmlUi pick through the 3D transform, mapped to
  surface-local, forwarded via wl_seat (criterion-3 pointer confirmed at edges).

Live update loop (was frozen on frame #1):
- send wlr_surface_send_frame_done to all mapped surfaces+subsurfaces each frame
  (client lives as an imported texture, not a wlr_scene node).
- re-import the surface's CURRENT buffer per commit, gated on the surface commit
  SEQUENCE not the buffer pointer (foot recycles a buffer pool -> same pointer,
  new contents); double-buffered wlr_buffer lock/release so the client is never
  starved. reimports now track commits ~1:1 (was capped 3:1).

--verify ALL PASS (criteria 1-7), kernel suite green, build + build-asan clean.
Spike target stays build_by_default:false, out of the shipped binary.

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