Function build_exchanged_claims 

pub fn build_exchanged_claims(
    params: &ExchangeParams<'_>,
) -> Result<ExchangedClaims, ExchangeError>

Build exchanged claims from a validated subject token. The resulting token preserves sub (the human), adds a new link to the act chain, scopes to the intersection of requested and subject scopes, and narrows audience to the target backend.