Networking checkpoint

Implement length-prefix framing on in-memory buffers (loopback-independent, deterministic): static byte[] EncodeFrame(string payload) — 4-byte little-endian length prefix followed by UTF-8 bytes; static string DecodeFrame(ReadOnlySpan<byte> data) — reads the prefix, validates that data.Length >= 4 + length (throw ArgumentException otherwise), decodes exactly length bytes. Then implement static async Task<string> EchoProtocol(string message): start a TcpListener on an ephemeral loopback port, accept one client, read one frame, echo the payload back framed, and return what the client received. The sandbox's loopback TCP is verified working.