Merge lane base check: develop 681ca9e2ace0

Merge lane base check: run the full gate on `develop` at `681ca9e2ace098f8581e5bf5468220ab365a7555` 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 certified: develop 681ca9e2ace0 green under the full gate (602s, 54,843 passed, 0 failed); certified event recorded, tree 3acfef1d. Nothing merged or ejected; queue empty. Memory: none applied.