Task: Currency Pair changes never publish NATS events (missing event registrar wiring)

Root cause of the "no eventing notification" gap noted repeatedly while manually testing Currency Pair this sprint (first surfaced during pair code derivation testing, again during duplicate pair_code testing): saving, editing, or deleting a Currency Pair never publishes ores.refdata.currency_pair_changed over NATS, so no client (including the one that just performed the save) gets notified to refresh.

projects/ores.refdata/service/src/messaging/event_registrar.cpp (the aggregator that wires each entity's generated <entity>_event_registrar.cpp=/=hpp into event_registrar::register_event_mappings()) only registers 6 entities: book, business_day_convention_type, country, currency, party_type, purpose_type. currency_pair (and currency_pair_convention) was never generated or wired — not even present in the file's own "stale drift, hand-wired in application.cpp instead" exclusion list, which documents ten other entities in that state. It simply fell through the cracks entirely: the domain event type (currency_pair_changed_event.hpp) and the Postgres NOTIFY trigger (refdata_currency_pairs_notify_trigger_create.sql) both exist and are codegen'd correctly, but nothing on the service side ever subscribed to the notify channel and republished it to NATS.

• [X] currency_pair_event_registrar.cpp=/=hpp generated via ores.cpp.nats-event-registrar and wired into event_registrar::register_event_mappings().
• [X] Saving a new Currency Pair, or editing/deleting an existing one, causes the list grid to refresh live (no manual Reload needed) — verified manually.
• [X] currency_pair_convention had the identical gap — fixed in the same pass (currency_pair_convention_event_registrar.cpp=/=hpp generated and wired alongside currency_pair's).

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Shipped: both entities' event registrars generated and wired into event_registrar::register_event_mappings(); confirmed via service log ("Registering event mapping: ores.refdata.currency_pair -> …" and the _convention equivalent) and manually verified live-refresh via Currency Pair: save/edit/delete refreshes the list live via NATS eventing — all steps pass for both entities. Root-caused via gdb-free log comparison against a working entity (currency), not guesswork.