Merge lane base check: develop 9718c0b4fc47

Merge lane base check: run the full gate on `develop` at `9718c0b4fc47c06afb1fd9854285e4479c72ef13` 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 9718c0b4fc47 certified green by merge_lane.py run: first full gate red on 1 API unit test, which passed alone and in a full rerun (58,204 passed), so the lane logged a flake. Queue empty, nothing ejected. Memory: none applied.