Function face_record_to_json 

pub fn face_record_to_json(rec: &FaceRecord) -> Value

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

Uses il/jl/kl as lb and ih/jh/kh as ub directly — no sorting. After face_matches_to_dict, these corners are physically corresponding: block1’s lb matches block2’s lb in xyz space.