Cut 2 adds a verify-envelope path beside the closure-token executor. The helper parses an envelope, binds it to the independently provisioned agent identity, validates time and lifetime bounds, selects the trusted key, verifies the signed contract, checks backend payload shape, and then refuses with backend_not_migrated.
Every path emits an unsigned MutationReceipt. No backend executes, no replay slot is consumed, and no artifact-custody claim is made until the first existing backend migrates.
320ad46e00