Merge lane base check: develop 7c78f09d81b8

Merge lane base check: run the full gate on `develop` at `7c78f09d81b87f22a1d6a413383ffd65b0224c1d` 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 of develop 7c78f09d81b8: merge_lane.py run (fleet copy; the repo has none) gated it with the full gate and reported certified. Hand cross-check: 207/207 tests pass. Nothing merged or ejected; develop is green.