refactor: improve code quality and add packet processing tests

- Convert iovec C-arrays to std::array<iovec, N> (4 sites in NUClearNet.cpp, 1 in header)
- Replace for(;;){break} in read_socket with while loop
- Split process_packet switch/case into if-else chain dispatching to per-type methods:
  process_announce_packet, process_leave_packet, process_connect_packet,
  process_data_packet, process_ack_packet
- Make per-type methods public for testability
- Add comprehensive behavioral tests (ProcessPacket.cpp) covering:
  validate_header edge cases, packet dispatch, connect handshake,
  data delivery/rejection/dedup/reassembly, ACK handling, send paths

Author: TrentHouliston