Video check: find what is wrong with a render before anyone else does

The person gets a video that has been checked like a professional editor checks an export, and a short REVIEW.md that says what was measured, what was looked at, what was fixed, and what nobody checked.

Three things are true the whole way through:

• Measure first, then look. A model looking at frames misses dead stretches and one-frame glitches; FFmpeg does not. Run check.py before you look, and never skip it because the video "looked fine".
• The person who made it is the worst reviewer. You know what each shot was meant to show, so you see the plan instead of the picture. Have someone without your plan look at the sheets (step 3).
• You cannot hear. The script measures loudness, peaks and silences. Whether the music fits, whether a sound is the right sound and whether speech is clear must be listened to by a person. Say so every time.

When the inputs are missing