Function direct_shell 

pub(super) fn direct_shell<const N: usize>(
    wave: Family,
    lattice: &BlochLattice,
    inputs: &Inputs<N>,
    shell: i64,
) -> Result<Jet<N>>

One unaccelerated cube shell of images, in the given lattice basis (upstream dsum*(..., i) with i = shell).

A 1D sum whose shift lies on the axis at half a period pairs the images direction * shell and -direction * (shell + 1), which lie at equal distances, so each shell adds a symmetric pair. That grouping jumps when the shift moves off the half cell, so jets fail there.