Merge lane base check: develop 42967d300887

Merge lane base check: run the full gate on `develop` at `42967d3008874e66adfbfed21ca4df17f9494a7e` 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`).

develop 42967d300887 is RED. The full gate failed on 3 Playwright specs (onboarding:51, tasks-bulk-select-rapid:83, worktrees-desktop:262), each failing 2 of 2 times when re-run alone. Reported develop_red; the control plane filed fix task #8209. Nothing merged and the lane is not held.