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 #6619 (task 8957) at 69e9318a. The quick gate passed on the merged tree (5963 tests). Nothing was ejected; the queue is empty, develop_red is null and there are no breaches. Batch took 50s (gate 34s). Memory: none applied.