Function derive_nonce_share 

pub fn derive_nonce_share(
    secret_share: &Scalar,
    message_hash: &[u8; 32],
    party_idx: u32,
) -> Scalar

Derive a deterministic nonce share for a party. The nonce is derived from the party’s secret scalar, the message hash, and their party index via HMAC-SHA256, reduced mod the P-256 group order.

The same inputs always produce the same nonce share — enabling deterministic signing and non-interactive nonce generation.