Async Engine.IO v4 client for Rust.

Engine.IO is the low-level transport protocol underneath Socket.IO. It negotiates a connection (HTTP long-polling or WebSocket), manages heartbeats, and delivers raw Message frames. Most users will never touch this crate directly; the higher-level sioc crate builds on top of it.

Architecture

┌─────────────┐
│   Engine    │
└──────┬──────┘
       │ spawns
┌──────▼──────┐
│  transport  │  (polling → optional WebSocket upgrade)
└─────────────┘

1. Engine::connect spawns the engine and transport tasks, returning channel handles for both.
2. The transport task performs the HTTP long-polling handshake and optionally upgrades to WebSocket transparently.