Function run_hd_bet 

fn run_hd_bet(
    magnitude: &[f64],
    grid: &Grid,
    params: &HdBetParams,
) -> Result<Vec<u8>, PipelineError>

Source the HD-BET weights and run it. Requires the onnx feature; weights come from the model registry (local $QSM_MODEL_DIR/cache, or the download feature).