Function lbv_core 

pub(crate) fn lbv_core(
    field: &[f64],
    mask: &[u8],
    grid: &Grid,
    tol: f64,
    max_iter: usize,
    progress: impl FnMut(usize, usize),
) -> (Vec<f64>, Vec<u8>)

LBV with an explicit iteration count.

Internal entry point; the public lbv wrapper supplies the default iteration count from LbvParams.