c49de5c500
Add TAG_SKELETON2D=6, SKEL2D_BYTES=819, decode_skeleton2D() to iphone_usb_bridge.py; handler in IphoneUSBSource writing persons_arkit_2d / persons_arkit_2d_t to state; new state fields in State dataclass. TDD: test_arkit_body.py (round-trip + wrong length → None). Import check verified.