Introduce a dormant MutationEnvelope beside the current closure token. Server, Agent, and helper now agree on length-prefixed manifest and authorization bytes, with one shared fixture pinning the hash, key identity, and Ed25519 signature.\n\nExecutor-affecting provenance, location, target, backend, and backend payload now have a signed home before any runtime path migrates. Exact duplicates stay visible in the hash; ordering does not. Current APT/DNF and Windows behavior remains untouched.
