Merge lane: jaah

Merge lane run: batch the queued PRs, run the full gate (if any) once on their merged tree, and merge the batch when it is green. Filed by the control plane (`app/merge_lane.py::ensure_lane`).

Merged #6146 (096a62a9b, quick gate green, 5853 tests). Ejected #6141: it conflicts with develop and needs a rebase. The develop_red record (vale, ast-grep pin) is stale: both stages pass on 096a62a9b. Queue empty. Time 43s: gate 32s.