Add host-to-npub static mapping with DNS hostname resolution

Add a HostMap that resolves human-readable hostnames to npubs,
enabling `gateway.fips` instead of the full `npub1...xyz.fips`.
Two sources populate the map: peer `alias` fields from the YAML
config and an operator-maintained hosts file at /etc/fips/hosts.

The DNS responder auto-reloads the hosts file on each request by
checking the file modification time, so operators can update
mappings without restarting the daemon.

- New src/upper/hosts.rs: HostMap, HostMapReloader, hostname
  validation, hosts file parser with auto-reload on mtime change
- DNS resolver checks host map before falling back to direct npub
- Node uses host map for peer display names
- Default hosts file added to both .deb and tarball packaging
- 26 new tests (789 total)

Cargo.toml

@@ -55,11 +55,12 @@ assets = [
     ["target/release/fipsctl", "/usr/bin/", "755"],
     ["target/release/fipstop", "/usr/bin/", "755"],
     ["packaging/common/fips.yaml", "/etc/fips/fips.yaml", "600"],
+    ["packaging/common/hosts", "/etc/fips/hosts", "644"],
     ["packaging/debian/fips.service", "/lib/systemd/system/fips.service", "644"],
     ["packaging/debian/fips-dns.service", "/lib/systemd/system/fips-dns.service", "644"],
     ["packaging/debian/fips.tmpfiles", "/usr/lib/tmpfiles.d/fips.conf", "644"],
 ]
-conf-files = ["/etc/fips/fips.yaml"]
+conf-files = ["/etc/fips/fips.yaml", "/etc/fips/hosts"]
 
 [dev-dependencies]
 tempfile = "3.15"