Fix DNS responder silent-drop on systemd-resolved deployments

The previous default configured systemd-resolved with `resolvectl dns
fips0 [<fips0_addr>]:5354`, intended to bypass an Ubuntu 22 systemd 249
interface-scoping bug. That target collides with the daemon's
mesh-interface filter on Linux: when an IPv6 packet's destination
belongs to a non-loopback interface, the kernel attributes the packet
to that interface in IPV6_PKTINFO (ipi6_ifindex == fips0) even though
loopback delivery is used (tcpdump shows lo). The mesh-interface filter
sees arrival_ifindex == mesh_ifindex and silently drops every query at
trace level — invisible to operators at the default debug level.

Net effect on stock deployments: every .fips query on systemd-resolved
hosts was silently dropped.

Daemon side
-----------

- Default `dns.bind_addr` changes from "::" to "::1" (IPv6 loopback
  only). The mesh-interface filter is then defanged on the default
  path because loopback isn't reachable from mesh peers. The filter
  remains in place defensively for operators who explicitly bind "::"
  to expose a mesh-reachable responder.

fips-dns-setup backend unification
----------------------------------

- New `try_global_drop_in` backend writes
  /etc/systemd/resolved.conf.d/fips.conf with DNS=[::1]:5354 and
  Domains=~fips. Inserted ahead of `try_resolvectl` in the dispatch
  chain. The standard loopback path has no interface scoping, so
  ipi6_ifindex reports lo and the filter passes.
- All other backends now target [::1]:5354 to match the daemon's
  default IPv6-loopback bind:
  - try_dns_delegate writes DNS=[::1]:5354
  - try_dnsmasq writes server=/fips/::1#5354
  - try_nm_dnsmasq writes server=/fips/::1#5354
- Fixed dns-delegate file path: was /etc/systemd/dns-delegate/, must
  be /etc/systemd/dns-delegate.d/ (with .d suffix). systemd-resolved
  silently ignored the previous path.
- fips-dns-teardown handles the new global-drop-in backend in cleanup.
- The legacy resolvectl per-link backend stays as a fallback,
  documented to require careful daemon bind_addr coordination.

fips-gateway upstream pairing
-----------------------------

- gateway.dns.upstream default changes from 127.0.0.1:5354 to
  [::1]:5354 to match the daemon's default bind. Linux IPv6 sockets
  bound to explicit ::1 do not accept v4-mapped traffic, so the old
  default would have caused the gateway's startup DNS reachability
  probe to time out and systemd to restart-loop the service.
- Operators who set a non-default daemon `dns.bind_addr` must also
  set `gateway.dns.upstream` to match — documented inline.

Documentation
-------------

- packaging/common/fips.yaml and packaging/openwrt-ipk fips.yaml
  examples updated; rationale for the bind_addr choice and the
  daemon/gateway pairing recorded inline.

Test coverage
-------------

- testing/dns-resolver/test.sh: real-fipsd end-to-end scenario added.
  Builds fipsd in a Debian 12 builder image (cached), runs the daemon
  with a real TUN in a privileged container, configures DNS via the
  setup script, and asserts `dig @127.0.0.53 AAAA <npub>.fips` returns
  AAAA. Refactored as a parameterized helper running across Debian
  12/13 and Ubuntu 22/24/26 (5 e2e scenarios). Backend-aware
  assertions: on systemd >= 258 the expected backend is dns-delegate;
  on older systemd it's global-drop-in. Strict content checks fail CI
  on any [::1]:5354 drift. fips-gateway also exercised in the
  debian12 scenario to lock the gateway-upstream pairing. Renamed all
  "fipsd" references to "fips" (project convention).

- testing/deb-install/ (new harness): builds the actual .deb via
  cargo-deb in a Debian 12 builder image (cached), installs via apt
  across each target distro, verifies maintainer scripts, conffile
  placement, binary placement, and end-to-end .fips resolution after
  start. Also exercises fips-gateway against the installed daemon to
  verify the gateway/daemon default pairing on a real .deb path.

- This is the test layer that was missing — the previous harness only
  verified config files were written, never that queries reached the
  daemon.

Verified: dns-resolver 78/78 assertions, deb-install 55/55 assertions
across all 5 distros (debian:12, debian:trixie, ubuntu:22.04,
ubuntu:24.04, ubuntu:26.04).

packaging/openwrt-ipk/files/etc/fips/fips.yaml

@@ -23,8 +23,15 @@ tun:
 
 dns:
   enabled: true
-  # bind_addr defaults to "::" (all interfaces, dual-stack).
-  # bind_addr: "::"
+  # bind_addr defaults to "::1" (IPv6 loopback). On OpenWrt, dnsmasq
+  # owns port 53; fips-dns-setup adds a forwarder rule pointing at
+  # 127.0.0.1#5354. Loopback bind is sufficient for that path.
+  #
+  # Set "::" to expose the responder to LAN clients directly (rare —
+  # most installs let dnsmasq forward instead). The mesh-interface
+  # filter in src/upper/dns.rs defends /etc/fips/hosts aliases from
+  # cross-mesh enumeration when bound to a wildcard.
+  # bind_addr: "::1"
   port: 5354
 
 transports:
@@ -79,6 +86,6 @@ gateway:
   lan_interface: "br-lan"       # LAN-facing interface for proxy NDP
   dns:
     listen: "[::1]:5353"        # gateway DNS listener (dnsmasq forwards here)
-    upstream: "127.0.0.1:5354"  # FIPS daemon DNS resolver
+    upstream: "[::1]:5354"      # FIPS daemon DNS resolver (matches daemon default)
     ttl: 60                     # DNS TTL and mapping lifetime (seconds)
   pool_grace_period: 60         # seconds after last session before reclaiming