Move mailboxes across hosts with execution-epoch fencing #4631

Agent pickup triage

Category: blocked. This classifies the next task, not completion or a verified current-source pass.

Next bounded step: Wait for #4630 and the transport contract, then implement one fenced migration state machine with crash/partition witnesses before expanding orchestration.

Read the linked handoff and coordinate any active owner before implementation. Preserve the original acceptance criteria below; this triage grants no spending, credentials, publication, host reconfiguration, or landing authority.

Parent: #4622

Depends on #4630 and #4632.

Outcome

Move a durable workload mailbox and its replayable state to a new host without allowing the previous execution to continue producing accepted messages, credential uses, or external effects.

Scope

• Monotonically increasing execution epochs and lease-based ownership.
• Source fencing before destination activation.
• Encrypted transfer of the signed snapshot, journal tail, delivery state, and required key-release evidence.
• Destination attestation and policy verification before activation in the sealed tier.
• Replay from the snapshot cursor and forwarding or rebinding of the stable mailbox identity.
• Broker enforcement that rejects credential and egress requests from stale epochs.
• Recovery from source crash, destination crash, partition, duplicate transfer, and interrupted handoff.

Acceptance criteria

• At most one execution epoch may produce newly accepted external effects after destination activation.
• Credential, egress, mailbox, and effect brokers reject a fenced or stale source epoch.
• Messages accepted before the cutover are either present after recovery or have a signed terminal disposition.
• Duplicate and interrupted handoffs reconcile idempotently without two active owners.
• Destination activation verifies snapshot integrity, journal cursor, policy, identity, and required assurance tier.
• End-to-end tests cover success, partition, source crash, destination crash, stale-source attempts, and replay after the last snapshot cursor.