Add external API integration for wiki/web UI synchronization

This commit adds a comprehensive external API system that allows external tools
(wikis, web UIs, etc.) to control and synchronize with Fantasy Map Generator.

New Features:
- External API Bridge (modules/external-api.js)
  - Event-driven architecture with EventEmitter
  - Map lifecycle control (create, load, save)
  - Data access methods (rivers, cultures, states, burgs)
  - Data mutation methods with auto-redraw
  - Export support (SVG, PNG, JSON)
  - Change detection with automatic event emission

- PostMessage Communication Layer
  - Auto-enables when FMG is embedded in iframe
  - Bidirectional message passing
  - Request/response pattern with promise support
  - Automatic event forwarding to parent window

- REST API Server (api-server/)
  - Express.js server with full CRUD operations
  - WebSocket support via Socket.IO for real-time updates
  - File upload support for map and CSV import
  - In-memory storage (can be replaced with database)
  - CORS enabled for cross-origin requests
  - Comprehensive endpoints for all map data

- Client Library (api-server/client.js)
  - Simple JavaScript client for REST API
  - Promise-based async methods
  - Works in browser and Node.js

- Demo Pages (demos/)
  - PostMessage integration demo with full UI
  - REST API demo with interactive testing
  - WebSocket demo for real-time events

- Documentation
  - Comprehensive integration guide (EXTERNAL_API_INTEGRATION.md)
  - API reference with TypeScript interfaces
  - Multiple integration examples
  - Troubleshooting guide

Integration Methods:
1. PostMessage Bridge - For iframe embedding
2. REST API - For server-side integration
3. Direct JavaScript API - For same-origin apps

Use Cases:
- Wiki pages that need to display and control maps
- Web UIs that want to edit map data
- External tools that need to sync with FMG
- Real-time collaborative map editing
- Batch operations and automation

Technical Details:
- Zero dependencies for external-api.js (pure JS)
- Auto-initializes on DOMContentLoaded
- Throttled change detection (500ms debounce)
- Deep cloning for data access (prevents mutations)
- Error handling throughout
- Version tagged (v1.0.0)

Updated Files:
- index.html: Added script tag to load external-api module

All APIs are backward compatible and don't affect existing functionality.

index.html

@@ -8098,6 +8098,9 @@
     <script defer src="modules/io/export.js?v=1.95.05"></script>
     <script defer src="modules/io/formats.js"></script>
 
+    <!-- External API for wiki/web UI integration -->
+    <script defer src="modules/external-api.js?v=1.0.0"></script>
+
     <!-- Web Components -->
     <script defer src="components/fill-box.js"></script>
   </body>