Select Clip Banner

Point an ALREADY-FINISHED clip at one of its existing burned banner variants (v2 §4.6). This is a pure pointer flip with no burn, no render, and no finishing quota spent. Name a variant from the clip's variants list, which the burn endpoint returns. 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 by naming a variant that exists on this clip. For service_unavailable, wait retry_after_sec and retry once.