Function hd_bet 

pub fn hd_bet(
    magnitude: &[f64],
    grid: &Grid,
    model_onnx: &[u8],
    params: &HdBetParams,
    progress: impl FnMut(usize, usize),
) -> Result<Vec<u8>, OnnxError>

Brain mask via HD-BET.

• magnitude — magnitude image, column-major (nx, ny, nz).
• model_onnx — bytes of the exported hd-bet.onnx (see crate::models).
• progress(done, total) — called after each network evaluation.

Returns a binary mask (0/1) on the input grid.