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 #6140 (8c7b7447) and #6145 (38e09cde), both quick-gated green (5850 passed). Ejected #6144: merge conflict. Stale develop red (vale, ast-grep pin) does not reproduce on head; both pass. Queue empty.