Correct transcript words

Correct what a clip's transcript says, one or more words at a time (at most 100 per request). Each edit addresses a word by its transcript index and sets either text (the corrected wording, which also unhides the word) or hidden (whether the word appears in captions and subtitles). Edits change the transcript, captions and subtitles only — never the audio or the clip's timing; use cut-by-transcript to remove spoken words from the video. Visibility and wording are independent: hiding a word preserves any correction made to it. The whole batch is applied atomically as a single transaction, so a request never changes only some of its words. A 500 does not by itself mean nothing changed — the words can land and a later step of the commit still fail — so treat the outcome as either fully applied or not applied at all. Every edit assigns a word outright, so re-sending the identical request is safe and settles it.