Merge lane base check: develop e4843268ab43

Merge lane base check: run the full gate on `develop` at `e4843268ab4356a2e156e41f13813985075460ee` 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 e4843268ab43 certified: full gate green on retry (61816 passed). First attempt's only red was a Runner CLI pins selftest timeout during a network stall, so the lane logged it as a flake. Nothing ejected. Memory: none applied.