Replace the trailer's black interludes.

Replaces the trailer's black interludes with the list given ([] removes them all; the beats read returns the current ones as interludes). A black interlude is a beat of pure black between two beats of the trailer (0.5 to 2 seconds), keyed by the beat it follows (afterBeatIndex, the beat's place from 0; at most one after any beat and never after the last), that may carry one sound (a breath, a bell) or one short line heard over the dark (at most 12 words), the narrator's or a character's own, always as a voice-over. It costs no render: the master draws the black and voices its line and its sound at no charge. Inserting, moving or removing a beat moves each black with the beat it follows. A whole list replaces the trailer's (an empty list removes them all); on a rendered trailer the master is stitched again with it, free. The whole list is validated: each follows a beat the plan has and never the last one, at most one after any beat, a line of at most 12 words, and a cast speaker who has a voice (400 otherwise). Free. On a draft plan the changed plan is queued for plan coverage again (planCoverage); in review or complete the master is re-assembled with the black, its lines and its sounds (reassembling). Refused while a render or finish pass runs (409). Requires Idempotency-Key. Scope: story:write.