Extract shared DNS wire-format helpers into src/derp/dns.py so both
the UDP plugin (dns.py) and the new TCP plugin (tdns.py) share the
same encode/decode/build/parse logic.
The !tdns command routes queries through the SOCKS5 proxy via
derp.http.open_connection, using TCP framing (2-byte length prefix).
Default server: 1.1.1.1.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>