0106 — Dropped lifecycle and lease machinery is parked as reference, not deleted

Context. The move to the append-only message_log plus cursor dropped the per-row status/attempts/lease_* columns, leaving the V1 ClaimMessages, backoff, ErrLeaseLost, and the commented Record* blocks referencing columns that no longer exist.

Decision. Keep that code parked rather than deleting it: the old lifecycle migrations move to migrations/old/, and the V1 claim, backoff, and Record* code stays in the tree as reference for when leases return. backoff showing as an unused-function lint is intentional.

Consequences. The tree carries dead code and a known lint on purpose, in exchange for not re-deriving the lease and backoff shapes when they come back. The parked code is not live — it references dropped columns — and is to be deleted or revived when leases land; backoff in particular returns with the exception-handling work.