Fix the header writer for a wire protocol

Your service sends a 6-byte header: a 32-bit sequence number, then a 16-bit port. The first version wrote the bytes in the host's native order, so it round-tripped in one process but every peer on another machine misread it. The fix writes both fields in network byte order, which is big-endian.

An assistant wrote write_wire_header(seq, port). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• seq: the sequence number, an integer.
• port: the port, an integer.
• Return the 6 header bytes as a list of integers, or [] when a value does not fit its field.

Rules the tests check:

• seq must be from 0 to 4294967295 (it fills 4 bytes) and port from 0 to 65535 (2 bytes). If either is out of range, return []. Never wrap a value to make it fit.
• Otherwise return the 4 bytes of seq, then the 2 bytes of port, each field most significant byte first.