Add D-180 EconEvent struct (target, effect, duration, visibility enums) with no-op handler to satisfy #809 spec. Import MARK_PRIMARY and MIXED currency zone assignments from wiki/economics/currency_zones.toml (D-172). All four D-179 stability tests now pass. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>