Function gmres_batch 

pub(crate) fn gmres_batch(
    rhs: &[Complex],
    columns: usize,
    options: GmresOptions,
    apply: impl Fn(&[Complex], usize) -> Result<Vec<Complex>>,
) -> Result<(Vec<Complex>, Vec<Convergence>)>

Independent GMRES solves of columns column-major right-hand sides. Only the operator applications are shared: each column keeps its own Krylov basis, rotations, restart decisions and true-residual convergence certificate.

The caller bounds columns to set the Krylov memory budget. apply receives only columns that still need that application, in column-major order.