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)
