CVE-2026-32129: Poseidon V1 variable-length input collision via implicit zero-padding

Poseidon V1 (PoseidonSponge) accepts variable-length inputs without injective padding. When a caller provides fewer inputs than the sponge rate (inputs.len() < T - 1), unused rate positions are implicitly zero-filled. This allows trivial hash collisions: for any input vector [m1, ..., mk] hashed with a sponge of rate > k, hash([m1, ..., mk]) equals hash([m1, ..., mk, 0]) because both produce identical pre-permutation states.

This affects any use of PoseidonSponge or poseidon_hash where the number of inputs is less than T - 1 (e.g., hashing 1 input with T=3).

Poseidon2 (Poseidon2Sponge) is not affected — it encodes the input length in the capacity element (IV = input_len << 64), making different-length inputs produce distinct states.