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 #6470 (Task 8500) as 4b001ee13 after the quick gate passed on the merged tree (5878 tests, batch 54s). Ejected none; queue empty. develop_red record at 5f25d1a is stale: the fix from Task 8499 landed after it; the post-merge base check confirms develop. Memory: none applied.