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.

api-server/package.json

@@ -0,0 +1,31 @@
+{
+  "name": "fantasy-map-generator-api",
+  "version": "1.0.0",
+  "description": "REST API server for Fantasy Map Generator with WebSocket support",
+  "main": "server.js",
+  "scripts": {
+    "start": "node server.js",
+    "dev": "nodemon server.js",
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [
+    "fantasy",
+    "map",
+    "generator",
+    "api",
+    "rest",
+    "websocket"
+  ],
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "express": "^4.18.2",
+    "cors": "^2.8.5",
+    "body-parser": "^1.20.2",
+    "multer": "^1.4.5-lts.1",
+    "socket.io": "^4.6.1"
+  },
+  "devDependencies": {
+    "nodemon": "^3.0.1"
+  }
+}