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 #5927, #5929, #5930 into develop (quick gate green, 5762 tests). Nothing ejected. Earlier red on develop came from a concurrent ~/.local package reinstall on the node, not from code. Deferred: selftest_runner fallback can't import tools/tests/support.