3-split water on molecule.js (Stage-1 port test). O is 3 third-split siblings sharing one +3 nucleus: sector 0 → bond to H1, sector 1 → bond to H2, sector 2 = lone pair (no H, points away from both H's). This is the split mechanism ported from mol_fast.js into the label-based box solver. If the lone-pair lobe appears and the 3 O-domains stay as 3 distinct wedges, the port works — then Stage 2 puts it in the moving liquid.