Function detect_edge_features 

pub fn detect_edge_features(
    verts: &[[f64; 3]],
    faces: &[[usize; 3]],
) -> (Vec<(usize, usize)>, Vec<(usize, usize)>)

Detect feature edges in a triangle mesh, classifying interior edges as convex (dihedral angle < π) or concave (dihedral angle > π).

Returns (convex_edges, concave_edges) as lists of (v0, v1) index pairs. Boundary edges (shared by only one face) are excluded from both lists.