Merge lane base check: develop 7d1d1440f01b

Merge lane base check: run the full gate on `develop` at `7d1d1440f01bb8bfb54357a1db5552298e3de2bc` 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).

Base check develop 7d1d1440f01b: certified green via merge_lane.py run (full gate, 57,845 passed). One API unit test flaked on run 1, passed in a focused rerun, and was reported as a flake. Nothing to merge, nothing ejected, queue empty. Memory: none applied.