Function classify_edge 

pub fn classify_edge(
    n0: [f64; 3],
    n1: [f64; 3],
    crease_angle_deg: f64,
) -> EdgeFeature

Classify an edge between two triangles by their dihedral angle.

n0, n1: outward normals of the two triangles. crease_angle_deg: threshold for classifying as a crease.