Merge lane base check: develop 8caa73985a2e

Merge lane base check: run the full gate on `develop` at `8caa73985a2e8ae4fd51304d4fac7475d6652c44` alone and report it certified or red: nothing had certified it since the last attempt. A red verdict files a fix task; it never holds the lane. Filed by the control plane (`app/merge_lane.py::_certify_idle`). How: run `python3 tools/merge_lane.py run` once. It takes this check's batch, gates that commit with the full gate in its sandbox and reports `certified` or `develop_red` itself. Never run the gate by hand: a hand-run green reports nothing, so no certificate is recorded.

develop 8caa73985a2e red: test_tenant_offboard census fails on 'email_mailboxes' (fails alone too); culprit 4b7ade5ce3 (Task 11433) unmapped the table without updating the offboard census. Memory: none applied