Function lookup_signer_for_chain 

pub(crate) fn lookup_signer_for_chain(
    from: Address,
    chain: u64,
) -> Result<TempoLookup>

Looks up a Tempo wallet signer scoped to the transaction chain.

Prefers an entry whose (wallet_address, chain_id) both match. Falls back to an entry with chain_id == 0 (the value when the field is absent) so that keys.toml files written by older Tempo clients (which omit chain_id) continue to work.