Merge lane base check: develop 686a2b39c7e7

Merge lane base check: run the full gate on `develop` at `686a2b39c7e76fd7e38d0e8955fde3d995cf7917` alone and report it certified or red: the lane was idle and nothing had certified it. Filed by the control plane (`app/merge_lane.py::_certify_idle`).