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 #6582, #6583, #6584 to develop (quick gate green on each merged tree, 5976 tests; batches 51s/49s, gate largest phase). 0 ejected. Stale develop_red at 6cbcaeda7 superseded by #6581 fix. Memory: none applied.