Merge lane base check: develop 210507de9f56

Merge lane base check: run the full gate on `develop` at `210507de9f56289177f2291c033da9a709851f4a` 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 210507de9f56: RED (lane reported develop_red). Full gate: 57716 passed, 1 failed, confirmed by focused rerun — test_cross_org_isolation::test_every_path_parameter_is_swept_or_named: new {signup_id} param from 95dd6fc0e (PR #6828) unswept. Nothing merged; queue empty.