feat: frame-paced compositor with sleep/wake back-pressure (~55 FPS)

Implement Wayland-style frame pacing for the GPU compositor. Client
windows now block in mark_window_dirty until the compositor consumes
their frame, providing natural back-pressure that locks animation to
the display rate.

Key changes:
- mark_window_dirty (op=15) blocks caller via scheduler sleep/wake
  (BlockedOnTimer with 50ms timeout fallback)
- composite_windows (op=16) wakes blocked clients via unblock() after
  GPU work completes
- BWM poll timeout reduced from 100ms to 1ms for responsive compositing
- Bounce uses delta-time physics (x1024 fixed-point) for FPS-independent
  animation speed
- Added partial TRANSFER_TO_HOST_3D for dirty windows only (~480KB vs 4.9MB)
- Early-out fast path when nothing is dirty (no GPU work)
- Z-order fix: page_phys_addrs always populated for registered windows

Result: 17 FPS -> 55 FPS, smooth animation, BWM/bounce balanced at ~33% CPU each.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ryanbreen