Task: Model a currency-country junction

Model a currency_country many-to-many junction between currency and the existing country entity (ores.refdata.country), following the same pattern as party_country_junction. A currency can be issued by multiple countries (e.g. EUR spans DE, FR, IT, …). This is independent reference data in its own right — not a prerequisite for the calendar entity, which carries its own country_code (with the ZZ sentinel handled in that task) rather than deriving a currency's calendars transitively through currency_country, since supranational calendars like TARGET have no single owning country to derive through.

• [X] A currency_country junction table links currency to country, generated via ores.codegen.junction, mirroring party_country_junction's shape.
• [X] Existing currencies are seeded with their real issuing countries (EUR -> DE, FR, IT, ES, …; USD -> US; GBP -> GB; etc.) for the currencies already present in synthetic/seed data.

(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.

Modelled currency_country as an ores.codegen.junction (SQL schema generated), and hand-authored the full C++ layer — domain struct, JSON I/O, table display/table I/O, custom generator, repository entity/mapper/repository — since junction codegen currently only supports the SQL address (confirmed by checking party_country and currency_currency_group, the two closest precedents: both are also SQL-only from codegen, with their C++ hand-authored). Mirrors party_country_junction's exact shape and conventions throughout.

Seeded 23 real currency-country mappings (EUR spans DE/FR/IT/ES/AT/FI; USD/GBP/JPY/CHF/CAD/AUD/SEK/NOK/DKK/CNY/HKD/SGD/ZAR/INR/KRW/MXN/BRL each map to one country) via both a generate_currency_countries() custom generator and a matching SQL populate script (refdata_currency_countries_populate.sql), following the same dual-path pattern established for calendar's QuantLib seed in the previous task. Discovered ES (Spain) wasn't yet in the countries seed (only calendar-referenced countries were there) and added it, since EUR -> ES is in this task's own acceptance criteria.

Wired the generated junction SQL into refdata_create.sql (was missing, same class of gap as calendar/calendar_type in the previous task) at the point after both currencies and countries exist.

Verified end-to-end: ores.refdata.api.lib=/=ores.refdata.core.lib build clean; full monorepo build and ctest (72/73 passed — the 1 failure is the same pre-existing, unrelated contact_type test seen in the previous task's PR, confirmed untouched by any commit here); compass db recreate runs clean with 23 junction rows present. Note: the real ISO currencies (EUR, USD, GBP, …) these rows reference are themselves populated via DQ-publish/tenant provisioning, not db recreate's foundation layer — a pre-existing characteristic of this codebase, not something this task changes.