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>
Test plan covering #806, #807, #808, #809 acceptance criteria aligned
with D-179 stability tests and Phase 2 economics deliverable requirements.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>