feat: advertise appliance names with mDNS

[castrojo]

What to build

Make the selected Bluefin Server appliance hostname discoverable on the local network as <hostname>.local using systemd-native multicast DNS. The first appliance should be reachable at blueserver-0.local, and the console should direct operators to its hostname-based KubeStellar URL.

Acceptance criteria

• The installed OS announces and resolves the selected hostname through mDNS without requiring a router DNS configuration or a separate DNS daemon.
• The wired appliance network uses mDNS and disables LLMNR.
• The resolver integration supports local .local lookups while retaining ordinary DHCP-provided DNS behavior.
• The console directs operators to https://<hostname>.local:8080/; blueserver.local is not introduced as an alias.
• Duplicate names are treated as an operator configuration error; the system does not auto-allocate a suffix or silently rename an appliance.
• Focused tests and a QEMU smoke check verify the default appliance address after the network is ready.

Blocked by

• feat: add persistent appliance hostname and Bash prompt #82

Design

docs/superpowers/specs/2026-09-09-appliance-identity-design.md