Add KuCoin WebSocket adapter for spot and futures with auth, batch subscriptions, and sandbox

[cto-new]

Summary

This PR implements a KuCoin WebSocket adapter that supports both spot and futures trading, including token-based authentication, automatic token refresh, and batch subscriptions. Public market data channels (ticker, order book, trades, OHLCV) and private account channels (balance, orders) are implemented. Sandbox/testnet support and robust error handling are included.

Details

• Implemented in src/websocket/adapters/kucoin.zig with an RPC-like subscribe/response model
• Added token-based authentication flow with periodic refresh
• Supported public channels: ticker, level2 (20/100), trades, OHLCV across multiple timeframes
• Supported private channels: balance updates and orders for authenticated users
• Added batch subscription and unsubscribe workflows
• Included sandbox endpoints support for spot and futures
• Added tests (kucoin_test.zig, kucoin_basic_test.zig, kucoin_integration_test.zig) and documentation
• Included performance considerations and memory-safety improvements
• Implemented robust error handling and reconnection logic