Function face_match_to_diagonal_json 

pub fn face_match_to_diagonal_json(fm: &FaceMatch) -> Value

Convert a FaceMatch to diagonal JSON format.

• In-plane (perm 0-3): block2’s lb/ub encodes direction. permutation_index: -1.
• Cross-plane (perm 4-7): ascending lb/ub. permutation_index: N (actual index).