Function poseidon2_hash 

pub fn poseidon2_hash(inputs: &[Fr]) -> Fr

Compute a Poseidon2 sponge hash over inputs.

Uses a rate-3 / capacity-1 sponge with the standard Aztec IV construction: state[3] = len * 2^64.

This matches barretenberg’s poseidon2_hash and the TS SDK’s poseidon2Hash.