Function repair_lost_share 

pub fn repair_lost_share(
    lost_participant: Identifier,
    helpers: &[(Identifier, KeyPackage)],
    pubkey_package: &PublicKeyPackage,
) -> Result<KeyPackage, ThresholdError>

Collaborative lost share reconstruction: allows a threshold subset of surviving signers to reconstruct a lost participant’s KeyPackage without ever reconstructing the group secret.