Merge lane base check: develop 321965c4968f

Merge lane base check: run the full gate on `develop` at `321965c4968f95df607a2f48cc66f93343c16d6b` 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 321965c4968f red (reported develop_red): access-tabs AC-7.2 and 2 nodes-roster-width 448px e2e tests fail on rerun; AC-8.3 flaked. No PR. Memory: none applied.