Merge lane base check: develop 515afd80ac74

Merge lane base check: run the full gate on `develop` at `515afd80ac74ae988a0fde6c91bcec43ad4086eb` 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 certified develop 515afd80ac74: full gate green (65,061 passed, 0 failed); nothing ejected, develop green, queue empty.