Merge lane base check: develop 3f9866b299c8

Merge lane base check: run the full gate on `develop` at `3f9866b299c83f85849826ca2638c1b7bdf98686` 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 3f9866b299c8: develop_red reported. Full gate 59111 passed, 2 failed deterministically (test_aws_ec2 bootstrap-fits-cap, test_virtual_nodes bootstrap byte-identical). Nothing merged/ejected; queue empty. Memory: none applied.