Function build_extended_grid_boundary_only 

pub fn build_extended_grid_boundary_only(
    data_energies: &[f64],
    resolution: Option<&ResolutionParams>,
) -> (Vec<f64>, Vec<usize>)

Build extended grid with boundary extension only (no intermediate points).

Used when the combined Gaussian+exponential kernel is active, where non-uniform spacing from adaptive intermediates degrades the Xcoef quadrature accuracy.