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 #6494 (Task 8565) at cd94e58c after the quick gate passed on the merged tree (5885 tests). Nothing ejected; queue empty. develop_red points at stale 5cd29f72, already fixed by Task 8600, so nothing was reverted. Base check 8601 still open. Batch took 52s, 35s of it gate. Memory: none applied.