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 #6382 (task 8224) and #6383 (task 8221) into develop; the quick gate on the batch tree was green (5878 passed). Nothing ejected; queue empty. The develop_red entry pointed at an old commit (42967d3) and was stale. Batch time 56s, gate 29s. Memory: none applied.