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docs/systems/_index.md

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+# Game Systems
+
+Core mechanics that govern how the game economy and logistics work.
+
+## Trade & Logistics
+- [Trade Routes](./trade-routes.md) - Automated ship loops between islands
+- [Diplomatic Trade](./diplomatic-trade.md) - Passive buy/sell with AI via treaties
+
+## Economy
+- [Maintenance Model](./maintenance.md) - Building upkeep and workforce costs
+- [Area Effects](./area-effects.md) - Proximity bonuses and penalties
+- [Statistics Screen](./statistics-screen.md) - Production vs consumption diagnostics
+
+## Infrastructure
+- [Shipyard](./shipyard.md) - Ship construction system
+- [Warehouse Cloud](./warehouse-cloud.md) - Shared island storage
+
+## Key Concepts
+
+### Maintenance
+Nearly every building has ongoing gold + workforce maintenance:
+- Fishing Hut: -6 gold/min, 4 Tier I workers
+- Bakery: -14 gold/min, 3 Tier II workers
+- Deleting buildings stops upkeep and frees workforce
+
+### Area Effects
+Buildings can give positive/negative proximity modifiers:
+- Shown by green/red arrows on placement
+- Spinner gives income boost near buildings
+- Polluters (e.g., Charcoal Burner) decrease happiness/health