Function isogeny_graph_distances 

pub fn isogeny_graph_distances(
    target: &Curve2,
    ell: u64,
    max_depth: u32,
) -> Vec<(Fp2, u32)>

Backward BFS on the supersingular ℓ-isogeny graph from target: the minimum ℓ-isogeny distance to each reachable j-invariant, out to max_depth. This is a sound per-digit oracle — a partial curve farther from the target than its remaining step budget cannot lie on the secret path, so its whole subtree is dead.