Function share_commitment 

pub fn share_commitment(
    blind: Base,
    c1_x: Base,
    c2_x: Base,
    c1_y: Base,
    c2_y: Base,
) -> Base

Native per-share blinded commitment:

share_comm = Poseidon(blind, c1_x, c2_x, c1_y, c2_y)

The y-coordinates bind the commitment to the exact curve point, preventing ciphertext sign-malleability. The blind factor prevents anyone who sees the encrypted shares on-chain from recomputing shares_hash and linking it to a specific vote commitment.