Function guided_chain 

pub fn guided_chain(
    roots: &[BigInt; 6],
    path: &[usize],
    p: &BigInt,
) -> GuidedWalk

Follow a guided chain of Richelot isogenies: apply the neighbour named by each index in path in turn, reporting the first split. In the full Castryck–Decru attack the path is not free — it is dictated by the torsion images φ(P), φ(Q): the kernel of each (2,2)-step is the image of a prescribed torsion subgroup, so the walk is a single guided line rather than the 15-way exhaustive tree. This function is that guided line; deriving path from the torsion images is the remaining Castryck–Decru integration.