previs-rerender: turn a locked rough cut into a finished treatment

Use a rough video as the structural reference for a new clip. The reference already decides the shot sequence, camera positions, edit rhythm, spatial relationships, and motion directions; the prompt changes the visual treatment without casually redesigning that structure.

This is a scenario layer over ofox-video-core. It owns the choice between a structure-preserving rerender and a continuation, the prompt contract, and the artifact review. The core owns credentials, request construction, pricing, submission, polling, download, error mapping, and the no-resubmit rule. Read the core's safety contract and approval-gate.md before a real run.

Safety and input contract