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 15 PRs (#6551–#6566) to develop, each quick-gated green on its merged tree. #6555 ejected once (mypy red on merged tree), merged after its fix. Queue empty; develop deploys green through ba1c58c4. Memory: used mem-18337 (inode check); none proposed.