Function sum_nonce_shares 

pub fn sum_nonce_shares(shares: &[Scalar]) -> Scalar

Sum T nonce shares (via Lagrange-weighted addition) to get the full nonce. In threshold signing, the full nonce k = sum of Lagrange-weighted nonce shares.

This is a simple sum — Lagrange weighting is applied at the signature level, not the nonce level, in most threshold ECDSA protocols.