Merge lane base check: develop 129323ac4ebc

Merge lane base check: run the full gate on `develop` at `129323ac4ebce8ef334b167439d107677ae91c8d` 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 129323ac4ebc certified: full gate green on the second attempt. The first attempt's only red stage, Markdown links, passed unchanged on rerun, so the lane recorded it as a flake. Nothing ejected. Memory: none applied.