Merge lane base check: develop 4624ecd01e48

Merge lane base check: run the full gate on `develop` at `4624ecd01e484244f254f0a181e54bf322c843cd` 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.

develop 4624ecd01e48 reported red: compose stack build failed because Dockerfile.api's support-kb stage was missing tools/check_docs_build.py. Already fixed forward on develop (cdda97f54d, 587db05d24); nothing to eject. Memory: none applied.