Function hash_two_to_quintic 

pub fn hash_two_to_quintic(a: Fp5, b: Fp5) -> Fp5

Hash a (Fp5, Fp5) pair as a 10-element preimage into a single Fp5.

Concatenates a.0 || b.0 and feeds the result through hash_to_quintic_extension. Used by Schnorr signing/verification (where the pair is (r, hashed_msg)) and by the L2 tx aggregation step (where the pair is (body_digest, attribute_digest)).