Merge lane base check: develop a977815e4971

Merge lane base check: run the full gate on `develop` at `a977815e497171278d4c146971d33fc1f4ed290c` 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 a977815e4971: RED (develop_red reported). Full gate 598s, 57228 passed, 2 failed deterministically: test_customer_billing.py payment_link recipients/billing-email tests. Queue empty; nothing merged or ejected. Memory: none applied.