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 #6531 (task 8720) at 29bc880d1 after a green quick gate on the merged tree (5888 tests pass). Nothing ejected; queue empty. develop_red points at stale c7b0ce8d4; base check 8723 on 5f27a552f still running. Batch took 64s, 45s of it the gate. Memory: none applied.