Function wigner_small_d_jet 

fn wigner_small_d_jet<const N: usize>(
    l: i32,
    m: i32,
    k: i32,
    theta: Jet<N>,
) -> Jet<N>

One small-d element d^l_(mk)(theta) at a jet angle, from the Jacobi recurrence (DLMF 18.9.1-2) in O(l) time and O(1) storage. Half-angle powers keep the off-diagonal limits where cos(theta) rounds to one. Symmetries keep both Jacobi parameters nonnegative.