Merge lane base check: develop d52906d0b9b0

Merge lane base check: run the full gate on `develop` at `d52906d0b9b0e12ec5465d3f5e69b483aa526d63` 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 d52906d is red, reported develop_red: test_credentials_connection_test.py::test_app_token_field_is_declared_but_no_row_ever_serializes_its_value fails on its own. 3 more API tests failed in the full run but passed alone (flaky). Nothing ejected.