Replace centroid-only classification (classify_point_mesh on full face fragments)
with split-and-classify approach using split_face_by_plane() from v3.3:
- Add include for brep_face_split.h
- Add face_normal() helper (extracts surface normal at parametric mid-point)
- Add split_and_classify_faces(): splits each face of body A by the planes of
every face in body B, then classifies each resulting fragment individually
- Refactor brep_union, brep_intersection, brep_difference to use the new
split-and-classify pipeline
- Add test: Union_SelfUnion_WithSplitting
This produces more precise boolean results by ensuring face fragments that
straddle the boundary are properly split before classification.
茂之钳