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 PR #6327 (Task 8138) → develop 19038334b after the quick gate passed on the merged tree (5883 tests); nothing ejected. Queued develop_red (mypy @55e8473) was stale: mypy re-run on develop is green. Queue empty.