Separate from internal/checker/socks5.go (different requirements: no
hex dumps, no diagnostic-friendly errors, faster path). Single Dial
entry point that handles greet + optional auth + CONNECT and returns
a ready-to-use net.Conn. UDP support deferred to P2.2.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>