Merge lane base check: develop d720b4f2bac9

Merge lane base check: run the full gate on `develop` at `d720b4f2bac90d7d7ac86df43e7bcba534a5faf9` 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.

Base check d720b4f2ba: develop_red, reported by merge_lane.py run. The mirror sensitive-content test failed because a stale baseline row was left by Task 10554. Fix PR #7124 passed the full gate but is superseded: develop 2306bb1f2e already has the equivalent fix (Task 10488), and the suite passes there. #7124 is left open, unmerged, with a comment. Memory: used mem-26916.