Task: Pilot server-side history diffs end to end on currency

Prove the server-side diff architecture end to end on one entity: currency. After this task the currency history dialog renders rows the server computed, with no comparison logic client-side.

• Currency history response carries fields and changes; old payload retained.
• CurrencyHistoryDialog holds no field-comparison code.
• Mapper and handler tests green; full build green; dogfood pass recorded.

1. Hand-write currency_field_mapper in ores.refdata.core: domain type → ordered field_value list ("ISO Code" → "AOA"); formatting per the conventions of the existing dialogs (Yes/No booleans, standard timestamp rendering).
2. Extend get_currency_history_response: per version add fields (mapper output) and changes (diff_result vs previous; empty for the oldest). Keep the full domain payloads during migration so other clients are untouched.
3. Handler composes: load versions → map each → diff consecutive pairs via ores.diff.
4. CurrencyHistoryDialog renders fields in the detail panel and changes in the changes tab; its calculateDiff retires.
5. Tests: mapper unit tests against the currency generator (field order, labels, each type's formatting); handler test via scoped_database_helper — write three versions, assert the changes rows between consecutive versions and the fields of the latest.
6. Dogfood: currency history dialog against a live backend before and after; identical visible behaviour.

• Dogfood (2026-06-05, live backend): bootstrapped a fresh environment, created currency ZZD and amended it twice (name, then symbol and fractions). currencies history ZZD in ores.shell — an untouched pre-pilot client — rendered all three versions correctly through the extended response, confirming the old payload is retained and the new handler path (mapper + diff compose) serves real traffic. Qt visual pass confirmed by Marco on a live desktop session: history dialog renders the server rows correctly (2026-06-05).

Merged in PR 1097. The server-side diff architecture is proven end to end on currency: each history version carries rendered fields (mapper order) and the field-level diff against its predecessor, computed by currency_field_mapper + the ores.diff engine in the handler; the full domain payload is retained for existing consumers. The Qt dialog is a pure renderer (server rows in the changes tab; a Field/Value details table) with no comparison code. Five mapper/compose tests plus a three-version repository round trip; dogfooded against a live backend via ores.shell (untouched client renders the extended response) and visually in the Qt UI by Marco. Review round 1 drew no comments on the pilot itself.