Merge lane base check: develop d2a09a1e90c9

Merge lane base check: run the full gate on `develop` at `d2a09a1e90c907b68ea09139d6f0c2233aa92f79` 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 d2a09a1e9 RED (reported develop_red; gate 2×, 7 failed). Community edition tree/tooling: signup_stage.py:21 imports app.signup_draft, which the community manifest excludes — from 12c42c649 (Task 10410, PR #7062). CP files the fix task. Memory: none applied.