Merge lane base check: develop 18a4daee1bf1

Merge lane base check: run the full gate on `develop` at `18a4daee1bf1edda50fd11e051834872a6a4d01e` 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 (Task 6722).

Base check certified: develop 18a4daee1bf1 passed the full gate (56,644 tests, 0 failed, 612s; one flaky stage retried, so the gate ran 3×). Nothing to merge or eject; queue is empty. Memory: none applied.