v0.19.0: Internal refactoring for improved maintainability

Internal improvements (100% backward compatible):

- Extract OfferPool class from Rondevu for offer lifecycle management
- Consolidate ICE polling logic into base RondevuConnection class
  (removes ~86 lines of duplicate code)
- Add AsyncLock utility for race-free concurrent operations
- Disable reconnection for offerer connections (offers are ephemeral)
- Fix compilation with abstract method implementations

Architecture improvements:
- rondevu.ts: Reduced complexity by extracting OfferPool
- connection.ts: Added consolidated pollIceCandidates() implementation
- offerer-connection.ts: Force reconnectEnabled: false in constructor
- answerer-connection.ts: Implement abstract methods from base class

New files:
- src/async-lock.ts: Mutual exclusion primitive for async operations
- src/offer-pool.ts: Manages WebRTC offer lifecycle independently

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/rondevu-client",
-  "version": "0.18.11",
+  "version": "0.19.0",
   "description": "TypeScript client for Rondevu with durable WebRTC connections, automatic reconnection, and message queuing",
   "type": "module",
   "main": "dist/index.js",