The bank slug, or null when the counterparty's bank is not a CFX partner.
Resolved alongside customerId so a selector trigger can show a name rather than an id.
This registration's row key in b2b.bank_account_view, so a caller can
look the account up there without reassembling the key — including when the
view resolves the key to a different arm, which is exactly the case a
caller cannot reconstruct on its own (§ 4.7).
Admin-tier projection of a
manual_bank_accountrow.idis the external id; every FK is projected as the referenced row's external id, so nothing internal leaks across the service boundary.