Merge lane base check: develop 9ae075f4b2d5

Merge lane base check: run the full gate on `develop` at `9ae075f4b2d56295b18b149db2a2c5f93d51cfa2` 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).

Base check develop 9ae075f4b2d5 certified green: merge_lane.py run full gate exit 0 in 620s, 55187 passed / 0 failed; event=certified (tree cd5dd8a2). No PR. Memory: none applied.