rlmesh_ext_box_elementwise_new
C-only extra: a Box with one bound pair per element (bounds_len must equal the shape's element count).
RLMeshStatus rlmesh_ext_box_elementwise_new(
const RLMeshBound* low,
const RLMeshBound* high,
size_t bounds_len,
const int64_t* shape,
size_t shape_len,
const RLMeshBoxOptions* options,
RLMeshBox** out
)