Function affine_from_quatern 

fn affine_from_quatern(q: &QForm, pixdim: (f64, f64, f64)) -> [f64; 16]

Rebuild the voxel→world affine a reader derives from a qform — the spec’s nifti_quatern_to_mat44. pixdim is pixdim[1..=3] as it appears in the header.