Beat-locked multicam editor. Takes N video sources synced to the same audio plus a beatlib BeatEvent sidecar, then rotates between cameras on each beat (or only on `is_downbeat: true` for slower cuts). Pure ffmpeg + beatlib, no ML deps. First consumer of @foenem_jarvis/beatlib v1.2 by a non-built-in producer pipeline. Use when the user has multiple takes of the same performance synced to one track and wants a music-video-style cut where the camera switches on the beat.