Function label_side 

pub fn label_side(atom: (f64, f64), neighbors: &[(f64, f64)]) -> LabelSide

Orient a label from the sum of neighbor vectors (SVG +Y down).

Matches RDKit getAtomOrientation (70° vertical slope; degree-1 prefers E/W; degree-0 defaults East except O/F/S/Cl-class → West).