Merge lane base check: develop 171d416138ee

Merge lane base check: run the full gate on `develop` at `171d416138ee65b57cd3e1d6273b9d7ca7be7518` 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).

develop 171d416 is red: the lane reported develop_red. The full gate passed 54,892 tests and failed 1 (529s): test_cross_org_isolation::test_no_route_lets_a_person_leave_an_org_themselves. The new DELETE /api/auth/signup/pending is missing from the census. It failed again when re-run alone. Nothing to merge. Memory: used mem-18424 (didn't run the gate on the same stack in parallel); proposed 0.