Function select_splitting_branch 

pub fn select_splitting_branch(
    branches: &[[BigInt; 6]],
    p: &BigInt,
) -> Option<usize>

The per-digit split oracle. Given the candidate surface for each branch of the recursive descent (one per guessed ℓ-adic digit — in the full attack each is built by Kani gluing from the torsion images and the guess), return the index of the branch whose surface splits. The split-test prunes every branch but the consistent one — the is_split/Kani step of Castryck–Decru, as a wired per-digit selector.