Story: Refdata entity NATS event registrar audit

Every refdata entity with a *_changed_event type (ores.refdata.api/eventing/) should have a working NATS event registrar (ores.refdata.service/messaging/*_event_registrar.cpp) wired into event_registrar.cpp, so downstream consumers (audit trail, cache invalidation, other services) reliably learn when the entity changes. Investigation at planning time found only 8 of 34 entities wired: book, business_day_convention_type, country, currency, currency_pair, currency_pair_convention, party_type, purpose_type. Since then, day_count_fraction_type (new entity, wired at commissioning) and contact_type (wired as part of a full model regeneration — see Commission: contact_type) have also been wired, leaving 24 of the original 26. The remaining 24 have a defined event type but no registrar — audit and close the gap for all of them. The registrar work itself is simple and mechanical per entity (mirrors the existing currency_event_registrar.cpp pattern); the value here is systematic completeness, not novel design.

Caveat (found in PR #1503 review): 10 of the 26 entities below — business_unit, counterparty, counterparty_contact_information, counterparty_identifier, currency_market_tier, monetary_nature, party, party_contact_information, party_identifier, portfolio — have stale changed_event.hpp output per Migrate remaining refdata entities to generated event-mapping registrar: the id field is the old plain ids instead of the current template's {{entity_singular_short}}_ids. Regenerating those 10 entities' nats-eventing output to add a registrar is therefore a breaking change to any existing consumer of the old field name — check every consumer (Qt controllers, handlers, anything referencing .ids on that entity's changed_event) before regenerating, not after. This applies to all 10 of those entities in the "Party family" and "Currency/FX family" task groups below, plus business_unit and portfolio in "Books/ portfolios/misc".

• Every entity in Entities missing a registrar below has a *_event_registrar.{hpp,cpp} following the existing currency_event_registrar pattern, registered in event_registrar.cpp.
• Each new registrar is exercised by at least one test confirming the NATS message is published when the entity changes.
• No entity with a *_changed_event type is left unwired by story close (bar any explicitly deferred and recorded under * Decisions).

Has a *_changed_event type but no *_event_registrar.cpp (24, was 26 — contact_type wired 2026-07-13): business_centre, business_unit, cds_convention, counterparty, counterparty_contact_information, counterparty_identifier, currency_group, currency_market_tier, currency_pair_classification, deposit_convention, fra_convention, ibor_index_convention, monetary_nature, ois_convention, overnight_index_convention, party, party_contact_information, party_identifier, party_id_scheme, party_status, portfolio, regulatory_book_type, rounding_type, swap_convention, zero_convention.

Planned tasks (not yet scaffolded as task docs) — one task per small batch of entities, grouped by domain family:

1. Party family: party, party_status, party_identifier, party_id_scheme, party_contact_information, counterparty, counterparty_identifier, counterparty_contact_information. Except party_status and party_id_scheme, every entity here has stale changed_event.hpp output (see Goal's caveat) — check all consumers of the old ids field before regenerating.
2. Currency/FX family: currency_group, currency_market_tier, currency_pair_classification, monetary_nature, rounding_type. currency_market_tier and monetary_nature have stale changed_event.hpp output (see Goal's caveat) — check consumers before regenerating.
3. Convention family: cds_convention, deposit_convention, fra_convention, ibor_index_convention, ois_convention, overnight_index_convention, swap_convention, zero_convention.
4. Books/portfolios/misc: business_centre, business_unit, portfolio, regulatory_book_type (contact_type already wired, 2026-07-13). business_unit and portfolio have stale changed_event.hpp output (see Goal's caveat) — check consumers before regenerating.
5. Add a test per new registrar confirming NATS publication on change (may fold into each batch's task above rather than a separate task, depending on effort).

• Adding *_changed_event types to entities that don't have one yet — this story only wires registrars for events that already exist.

• Migrate remaining refdata entities to generated event-mapping registrar — the 10-entity stale-changed_event.hpp caveat in the Goal above; check this capture before regenerating any of its 10 named entities.