Under --iphone-usb, skip all three MediaPipe detectors
(pose, face, hand) and rebuild the MP-33 body from
state.persons_arkit_{2d,joints} via new arkit_body_2d /
arkit_body_3d builders. Add head anchor (ARKIT_HEAD_IDX=51,
needs live verify) as MP33 slot 0. Webcam path unchanged.