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 #6677 (task 9143) as 3e6be77 after the quick gate passed on the merged tree (6014 tests). Nothing ejected. Queue empty, develop not red. Batch time 56s, of which the gate took 35s. Memory: none applied.