Merge lane base check: develop 4e8587155fa5

Merge lane base check: run the full gate on `develop` at `4e8587155fa53029007611b200cd3b85004a39b4` 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 4e8587155fa5 certified green on the full gate; 1 API unit test flaked on the first run, passed alone and on the full rerun. Nothing ejected.