Detect AddrInUse on bind, print a human-readable diagnostic explaining
systemd-resolved / Dnscache as the likely cause and offer two concrete
fixes (sudo numa install, or bind_addr on a non-privileged port), then
exit(1) instead of surfacing a raw OS error.
Adds tests/docker/smoke-port53.sh: end-to-end Docker test that
pre-binds port 53 with a Python UDP socket and asserts the advisory +
exit code.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
