Review 004 items 10+11: the two recorded mem growth postures
(subscribe backlog drain unbounded on the consumer side; O(live-rows)
claim scan with full-row clones) and the outbox's 'processed means
delivered-or-reclaimable' semantic, pinned in engine-mem.md. The
debug_assert arm was declined: the dropped ack/retry booleans also
refuse benignly on legitimate in-closure handle ops, so it cannot
separate a visibility lapse from a deliberate first-party outcome.