fix(video): every caption rendered with a leading comma

The short-form caption file was shipped with a broken Events Format line: it
named MarginL and MarginR but not MarginV, while each Dialogue line emits three
margin values (0,0,0). libass read the third as Effect and folded the comma that
followed into the text, so every burst rendered as ",Starknet can hide who"
instead of "Starknet can hide who".

Invisible in the file, invisible in the generator's own checks, visible only
once the .ass was actually burned into a frame with ffmpeg and the pixels were
read back. Found by rendering a test clip and looking at it.

MarginV is restored, and a guard now asserts the Format's field count matches
what each Dialogue emits, since this was exactly the kind of desync that hides
until render time. Removing MarginV from either side now fails the generator.

Confirmed fixed by re-rendering: the caption reads "Starknet can hide who",
bottom-centred, white on a heavy outline, no comma.