WebSocket Server

# WebSocket Server PRD

## Overview
Build a WebSocket server from raw TCP sockets implementing the full WebSocket protocol including HTTP upgrade handshake, binary frame parsing, and a pub/sub room system for real-time messaging.

## Requirements
- HTTP upgrade handshake: parse the Upgrade header and compute Sec-WebSocket-Accept correctly
- WebSocket frame parsing: opcode, mask bit, payload length (7-bit, 16-bit, 64-bit), masking/unmasking
- Support both text and binary message frames
- Handle fragmented messages with continuation frames reassembled into complete messages
- Ping/pong heartbeat mechanism for connection keep-alive
- Close frame handling with status code and reason phrase
- Pub/sub rooms: clients can join rooms, leave rooms, and broadcast messages to all room members
- Per-connection state storage for application-level data
- Connection lifecycle hooks: onConnect, onMessage, onClose, onError
- Broadcast to all connected clients across all rooms
- Send a message to a specific connection by its unique ID
- Message rate limiting per connection to prevent abuse
- Maximum message size limit with rejection of oversized frames
- Graceful shutdown that sends close frames to all connections before exiting
- Support 100+ concurrent connections without performance issues
- Integration test demonstrating communication with a standard browser WebSocket client
- CLI to start the server on a configurable port

## Tech Stack
- TypeScript / Node.js
- Raw TCP sockets only (net module) — no ws, socket.io, or WebSocket libraries
- Crypto module for SHA-1 handshake computation

## Scoring Criteria
- **Functional (40%)**: Handshake completes, frames parse correctly, messages send and receive
- **Quality (20%)**: Proper protocol compliance, clean connection management, error resilience
- **Fidelity (25%)**: All features including rooms, fragmentation, rate limiting, and graceful shutdown
- **Speed (15%)**: Time bonus