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 #6524 (task 8692) at 29d93a4b after a quick gate on the merged tree (5897 passed). Nothing ejected; queue empty. develop_red points at the older 9b6817e9; base check 8690 owns that. Batch took 49s. Memory: none applied.