blank: pivot at center (z=0) so flat laptop is symmetric from any angle
- Base extends forward (z: 0 to 2*hd), lid at 180° extends backward (z: 0 to -2*hd)
- Both top surfaces at y=0, symmetric about pivot — looks level from any camera tilt
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>