A framed echo server

1. static String frame(String message) — return the wire form: message + "\n". 2. static java.util.List<String> deframe(String received) — split a received chunk into complete frames: split on "\n" and DROP a trailing fragment that lacks its terminator (an incomplete frame is not a frame). Empty input → empty list. 3. static byte[] lengthFrame(String payload) — binary framing: 4-byte big-endian length prefix followed by UTF-8 payload bytes. 4. static int prefixLength(byte[] framed) — read the 4-byte big-endian prefix back.