Function find_split_neighbour 

pub fn find_split_neighbour(
    roots: &[BigInt; 6],
    p: &BigInt,
) -> Option<[Quad; 3]>

Walk the (2,2)-isogeny graph from a genus-2 Jacobian given by its six Weierstrass points: try each of the 15 Richelot neighbours and report whether any is a split (reducible) surface. This is the shape of the Castryck–Decru search — at real scale it is guided by the torsion images rather than exhaustive, and Robert’s generalisation lifts the same walk to dimension 4/8. Returns the splitting quadratics of the first split neighbour, if one exists.