Merge lane base check: develop 903e043ac9aa

Merge lane base check: run the full gate on `develop` at `903e043ac9aad7a33757c063e0dd7af5a2e4ef4e` 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).

develop 903e043ac9aa certified green by merge_lane.py run: full gate 809s, 56,729 passed / 0 failed (2 gate runs; the first red run was recorded as a flake). Nothing ejected; no fix task needed. Memory: none applied.