Merge lane base check: develop 8e3978015de9

Merge lane base check: run the full gate on `develop` at `8e3978015de9f2f89c6c473fd508e482b453a1a7` 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 8e3978015de9 certified green via merge_lane.py run (full gate, 708s, 57857 passed/0 failed; tree 1fe8d30c). Queue empty, no breaches. Nothing merged or ejected. Memory: none applied.