PREFERS OFF TRACK

Compares best_bris_speed_off_track against best_bris_speed_fast_track. A 5-point differential is a meaningful signal that the horse's physical makeup — hoof shape, stride mechanics, or ground feel — is better suited to soft, muddy, or sloppy conditions. This is a speed-based off-track preference signal and is distinct from OFF_TRACK_SPECIALIST, which is based on win rate. PREFERS_OFF_TRACK can fire even on horses with limited wet starts, as long as the speed differential is present.