Merge lane base check: develop 409663b7eb71

Merge lane base check: run the full gate on `develop` at `409663b7eb71dc6712d2e87435c7de04aad6dd9c` 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).

develop 409663b7eb71 certified green: merge_lane run gated tree e27513ff12 (exit 0), event=certified. Nothing merged/ejected. Lane printed "0 tests" (counter quirk); sanity run showed 154/154 pass. Memory: none applied.