feat(headtrack): move head tracking into engine, add IMU diagnostics

feat(headtrack): move head tracking into engine, add IMU diagnostics

- drop product-side headtrack classes; the engine now auto-enables
  hot-plug head tracking for every app (Workspace delegates
  getHeadTracker() to ViewPanel)
- add --headtest and --headdebug diagnostic modes to Main: live
  IMU angle dump and a periodic stream-health line (frames, tick,
  dt, max gyro, calibrated angles)
- fix terminal fg/bg channel order in TerminalPanel cell caching;
  add PtySession.clear()/fillBox()
- document the head tracking boundary exception in doc/index.org