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 #6483 (Task 8583) as 5cd29f72 after the quick gate passed on the merged tree (5889 tests); nothing ejected and the queue is empty. #6483 fixes the operator-dashboard e2e test that had develop red; the base check will confirm. Batch time 48s, 33s of it the gate. Memory: none applied.