Move behavior: snap the dragged selection's union box (edges + center) to
candidates, shaping the proposed translate. The gesture applies the
transform uniformly to every dragged id, so the selection shifts together
and stays rigid. Single-select is the degenerate one-box union. Publishes
the matched line(s); clears on miss/end.
Move behavior: snap the dragged selection's union box (edges + center) to candidates, shaping the proposed translate. The gesture applies the transform uniformly to every dragged id, so the selection shifts together and stays rigid. Single-select is the degenerate one-box union. Publishes the matched line(s); clears on miss/end.