Replace the resolvectl-only fips-dns.service with a detection script
that configures whichever DNS resolver is available:
1. systemd dns-delegate (systemd >= 258, declarative drop-in)
2. systemd-resolved via resolvectl (most systemd distros)
3. dnsmasq (standalone)
4. NetworkManager with dnsmasq plugin
5. Warning with manual instructions if none found
Service reloads are non-fatal — config is written and the backend
is recorded even if the reload fails, preventing state file cleanup
issues under set -e.
Teardown reads the recorded backend from /run/fips/dns-backend and
reverses the configuration, or cleans up all possible backends if
the state file is missing.
Includes a Docker-based test harness (testing/dns-resolver/test.sh)
covering all five backends across Debian 12, Debian 13, Fedora,
and bare systems.
Fixes#52.
