Reorganize src/ directory into feature/domain-based structure

Restructure flat src/ directory (17 files) into organized folders:

Structure:
- src/core/ - Main API (rondevu.ts, offer-pool.ts, types.ts, index.ts)
- src/connections/ - WebRTC connections (base.ts, offerer.ts, answerer.ts, config.ts, events.ts)
- src/api/ - HTTP layer (client.ts, batcher.ts)
- src/crypto/ - Crypto adapters (adapter.ts, node.ts, web.ts)
- src/utils/ - Utilities (async-lock.ts, exponential-backoff.ts, message-buffer.ts)

Changes:
- Move all 17 files to appropriate feature folders
- Update all import paths to reflect new structure
- Update package.json main/types to point to dist/core/index.js
- Preserve git history with git mv

Benefits:
- Clear separation of concerns
- Easier navigation and maintenance
- Better scalability for future features
- Logical grouping of related files

🤖 Generated with Claude Code

package.json

@@ -3,8 +3,8 @@
   "version": "0.20.0",
   "description": "TypeScript client for Rondevu with durable WebRTC connections, automatic reconnection, and message queuing",
   "type": "module",
-  "main": "dist/index.js",
-  "types": "dist/index.d.ts",
+  "main": "dist/core/index.js",
+  "types": "dist/core/index.d.ts",
   "scripts": {
     "build": "tsc",
     "typecheck": "tsc --noEmit",