Merge lane base check: develop 86fb595444b6

Merge lane base check: run the full gate on `develop` at `86fb595444b681f276deb4e3681a0b6de5f38662` 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 86fb595: recorded develop_red. The cause is the environment, not the code (0 failed tests): mypy INTERNAL ERROR from a corrupt lane mypy cache, and a cold cache passes. Cache cleared, nothing merged or ejected. Memory: used mem-23540; proposed 1 gotcha (mem-24221).