arena: decouple player rotation from camera rotation
Introduce playerFacing as independent state. During normal camera look-around,
playerFacing tracks cam.rotY. During right-click orbit, playerFacing stays
frozen so the player body doesn't spin.
This allows the camera to swivel freely around a stationary player during
orbit, enabling true freelook without player model rotation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>