Composing PR-4 (intake/delivery split) with PR-5 (persisted transport_profile): the delivery
fallback to the runtime profile's unique adapter is only for sources with NO identity. A pinned
identity names the receiving bot; if that bot has no adapter it is offline and the lane fails
closed, never answering from the runtime profile's bot. Also: tests and docs reference the split
helper, not the removed _adapter_for_source.