Post a swarm worker's stack and receive its receipt

Requires a run capability carrying the swarm.submit_stack operation. One receipt per attempt: an identical replay answers 200 with the existing receipt, a different payload for the same attempt is 409 stack_payload_conflict, and a new payload behind the swarm's generation is 409 attempt_fenced. Bodies over 25 MiB, declared or decoded, are 413 stack_too_large before any receipt exists. input_revision and notice target_tip values are original 40-character Git commit IDs. Change keys retain their stable 32-character hex or reverse-hex spelling; SHA-256 evidence digests remain 64-character hex values.