Merge lane base check: develop a4d3f61c1a49

Merge lane base check: run the full gate on `develop` at `a4d3f61c1a49b3eb5a6748cf1906e19751d2136a` 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 a4d3f61c1a49 certified: merge_lane.py run full gate green in 672s (57,860 passed, 0 failed), event=certified. Nothing merged or ejected; develop green. Memory: none applied.