Add modules/hostel-wifi.nix: per-SSID MAC randomization (iwd), disable
mDNS/LLMNR/NetBIOS, reject hostile IPv6 RAs, firewall LAN-side probing,
an nftables killswitch target allowing only tunnel egress + captive-portal
endpoints, NTS-secured time via chrony, and a captive-portal helper.
Import left commented out; toggle via 'untrusted on|off'.
