Function face_match_to_json 

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

Convert a FaceMatch to JSON with directed lb/ub corners.

Both block1 and block2 export raw lb/ub (il/jl/kl → ih/jh/kh). The permutation_index is included for reference but the corners already encode the full mapping: lb1 ↔ lb2, ub1 ↔ ub2 in physical space. The optional permutation_matrix (2x2 i8) is exported when present so downstream consumers (e.g. glennht-core) can use the declarative matrix-driven path instead of the bare index.