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 8 PRs (#6436, #6439, #6440, #6443, #6444, #6445, #6446, #6447) to develop; none ejected. Each batch quick-gated green (5878 passed); develop base gate green. Queue empty. Memory: none applied.