Create Clip Banner

Burn ONE banner onto an ALREADY-FINISHED clip's vertical master (or its 16:9 render) and select it (v2 §4.6). The text is server-resolved from the clip's persisted hooks by lane and rank; client-supplied text is ignored. This does not consume finishing quota, because a burn is CPU-only, and an already-burned variant is reused at $0 (reused:true). Recover from scope_missing by minting a finish:write key, run_not_ready by polling progress until the run completes, run_not_found or clip_not_found by listing your runs and their clips, and validation_error (an unknown hook, or a basic-family style whose banner is engine-owned) by fixing lane, rank, or target. For rate_limited or service_unavailable, wait retry_after_sec and retry once.