Function small_split_kambe 

pub(super) fn small_split_kambe<const N: usize>(
    n: i32,
    z: &Jet<N>,
    kambe: &SmallSplitKambe,
    largest: &Rounding<N>,
    bound: &mut Rounding<N>,
    factor: &Jet<N>,
) -> (Complex, Option<Complex>)

I_n and, for jets, I_(n+2) at z in RealKambe::SmallSplit, storing their bounds in the term with the factor factor in bound.

Kept out of line, like Shells::add_compensated, so the term loop at the other splits stays small. Calibrated: inlined, it slows 3D jets at the automatic split by 1% to 2.7%.