Merge lane base check: develop b954e5cfa553

Merge lane base check: run the full gate on `develop` at `b954e5cfa5530b9e7f50f9e35b86b4a4627366d8` 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 b954e5cfa553 certified by full gate (65,051 passed); one API-unit flake recorded on first run, cleared by focused rerun and green third run. Nothing ejected, queue empty.