Merge lane base check: develop 5e32e975be6c

Merge lane base check: run the full gate on `develop` at `5e32e975be6c6760b028bfda101d8b42c03f5f39` 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 5e32e975be6c reported red, but it is a false red: only mypy failed, with an INTERNAL ERROR from a stale incremental cache in the lane's gate venv (app.forge_repos). I cleared that cache and mypy is now green in the sandbox. The lane won't re-gate this sha. Memory: proposed 1 gotcha (mem-31545).