spike(kernel): --demo perf scenario — 4 corner windows + FPS HUD/log

New `--demo` mode for a real-seat perf load test (the Phase-0 perf measurement):
- Four windows, one per screen corner, angled INWARD (perspective + per-corner
  rotateX/rotateY) — a curated layout, distinct from --run's single centered
  window.
- Clients: 3x foot + 1x vivaldi (Chromium on Wayland via
  --ozone-platform=wayland; probes vivaldi-stable/vivaldi/vivaldi-snapshot,
  loud warning if absent). Replaces firefox, which rendered as an empty/clear
  box on this compositor. Each client claimed into the next free corner slot at
  map time (browser steered to its designated corner via app_id match;
  vivaldi/chromium/firefox all recognized); per-slot bookkeeping freed on
  unmap/destroy.
- Live FPS HUD (RmlUi text) in the corner; per-5s min/max FPS written to
  $UNBOX_SPIKE_FPS_LOG (default $HOME/rml-spike-fps.log), fflush+fsync per line.
- Demo dead-man defaults to 120s (P resets; Esc/Ctrl+Alt+Backspace quit) so an
  HD video can play while watching FPS.

All existing behavior intact (live-update loop, input routing through the
transform, cursor, escape hatch, persistent log). --verify ALL PASS, kernel
suite green, spike builds clean (build_by_default:false, out of the shipped
binary). Headless smoke: 3 foot + vivaldi map into 4 distinct corners; 5s
min/max FPS lines written.

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