feat: add Durable Objects pattern for real-time state management

[talkstream]

Summary

• Implement base Durable Object with storage abstraction for platform-agnostic usage
• Add WebSocket room for real-time collaboration features
• Create distributed counter and named counters with atomic operations
• Implement rate limiter with multiple algorithms (fixed window, sliding window, token bucket, leaky bucket)

Implementation Details

Base Durable Object (BaseDurableObject)

• Provides common functionality and state management
• Includes helper methods for storage operations (get, put, delete, list)
• Implements response helpers (json, error, success)
• Includes SimpleState for testing outside Cloudflare environment

WebSocket Room (WebSocketRoom)

• Real-time communication capabilities
• User management with metadata
• Message history and broadcasting
• Auto-cleanup on disconnect

Distributed Counters

• Counter - Single atomic counter
• NamedCounters - Multiple counters in one object
• HTTP API for increment/decrement/reset operations

Rate Limiter

• Fixed window rate limiting (basic)
• Sliding window algorithm
• Token bucket algorithm
• Leaky bucket algorithm
• Accurate distributed rate limiting

Test Coverage

• Comprehensive tests for base class
• Counter operations tests
• Storage persistence tests
• Transaction support tests

Documentation

• Detailed guide in docs/DURABLE_OBJECTS.md
• Working example in examples/durable-objects-example.ts
• README updated with Durable Objects section

Use Cases

This enables building:

• Chat rooms and collaborative editing
• Real-time game servers
• Distributed systems with strong consistency
• Global rate limiting
• Event aggregation
• Leader election patterns

Performance Benefits

• Edge location execution for low latency
• Strong consistency guarantees
• Automatic scaling and hibernation
• Single-threaded JavaScript execution
• Transactional storage operations

Test Plan

• Run all tests locally
• Verify TypeScript strict mode compliance
• Check ESLint passes with no errors
• Deploy example to Cloudflare Workers
• Test WebSocket connections
• Verify rate limiting accuracy
• Test counter persistence