Function generic_third_contracted 

pub fn generic_third_contracted<const K: usize, P: RowNllProgramGeneric<K> + ?Sized>(
    prog: &P,
    row: usize,
    dir: &[f64; K],
) -> Result<[[f64; K]; K], String>

Evaluate a generic program at the one-seed scalar crate::jet_scalar::OneSeed, returning the contracted third Σ_c ℓ_{abc} dir_c — the row_third_contracted(dir) channel — WITHOUT materialising the dense t3 tensor. The contraction direction is folded INTO the differentiation by the nilpotent ε seeded with dir.