Function bdiv_periodic_inplace_f32 

pub(crate) fn bdiv_periodic_inplace_f32(
    div: &mut [f32],
    gx: &[f32],
    gy: &[f32],
    gz: &[f32],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f32,
    vsy: f32,
    vsz: f32,
)

Backward divergence with periodic boundary conditions (f32, in-place) Adjoint of fgrad_periodic_inplace_f32, matching MATLAB’s gradfp_adj_mex. At boundaries, wraps around: at i=0, uses gx(end) instead of zero.