Score one scene's music bed.

One endpoint, selected by action: score generates a new bed from a prompt, apply-library reuses a track already in My Library (no generation, and the way several scenes come to share a tempo and key), update relevels or retimes the bed, remove clears it. Beds render with the Lyria engine, so they are never blocked by the fal balance; describe the SOUND rather than naming a real artist. Generating — score, or an update that changes the prompt or vocals — waits for the project's gate: while it is closed the call is refused with 409 project_precondition_failed and no bed is written or queued; apply-library, a level or timing update and remove stay open. Requires Idempotency-Key. Scope: story:write.