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 #6503, #6504, #6505, #6506 into develop (quick gate green, 5886 tests). None ejected. Queue empty, develop_red null, no health breaches. Batch time 45s: gate 30s, merge 6s, admit 4s. Memory: none applied.