Provision an identity-blind FBO pool virtual account (partner perimeter).

Creates a persistent virtual account attributed to the authenticated partner. The partner identity (req.user.partnerId from PartnerHmacGuard) is recorded on bankrail.virtual_accounts.consumer_partner_id; the callback dispatcher will sign every credit-confirmed callback to this VA with the partner active callback-signing credential when one exists. Returns 200 OK with the allocated VA details (synchronous creation — bankrail calls Providus DCS during the request). BRR-bearing is governed by the partner customer class (the substrate rule). Government and corporate partners receive BRR-bearing VAs by default; fintech and citizen partners receive reference-only VAs (the brr field is null on the response). The response shape is identical across all classes; only the brr field differs. Fintech opt-in to BRR-bearing is a named future unit.