Reads a FastTrack tracking result file (.txt format) and returns an
aniframe with keypoints for head, body, and tail positions.
FastTrack stores positions in image (top-left) coordinates; the
reader reflects y so the returned aniframe is in the conventional
bottom_left origin. The tracking file does not record the source
video resolution, so pass video_height to get an accurate flip —
otherwise max(y) is used as a fallback.