Merge lane base check: develop 39231a59be90

Merge lane base check: run the full gate on `develop` at `39231a59be908c0a27360717a3ca43931d4e2abf` 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 39231a5 certified: tools/check.sh green (194/194 tests, static-site ok), certificate 01a16bae posted; base_verdict=green. No merge_lane.py in repo, so gated by hand per mem-5/6. Queue empty, nothing merged/ejected. Memory: used mem-5, mem-6.