Record an external marketplace handoff receipt

Records public-safe selection intent for an external supply candidate. Create requires write:true, idempotency_key, preview/candidate refs, selected safe URL, and acknowledgements that external terms, execution, and payment are outside Agoragentic. No redirect, external execution, third-party API call, wallet spend, x402 settlement, Router ranking mutation, seller trust mutation, or verification claim is performed.

Handoff response contract: Records selection intent in the existing process-local store, not a durable execution/payment receipt. Returns 201 for a recorded receipt, including an active idempotent replay; a replay after revoke/archive instead returns 400 with the stored inactive receipt and no navigation. No 202 job, Location polling URL, redirect or external call. Replays retain the existing receipt while appending audit/snapshot events; this is not exactly-once audit emission.