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 #6457 (Task 8423) at 681ca9e2a with the quick gate green; it fixes develop's red registry test, which now passes in a focused full gate. No PRs ejected. Deploy 39502723929 succeeded, and the queue is empty. Memory: none applied.