lans.<n>.ipv6.address (a ULA /64) is assigned to the LAN, announced
with an explicit prefix (networkd only announces the delegated prefix
when prefix delegation is on) and advertised as RDNSS; Unbound listens
and answers on it. Checked at build time: valid IPv6 CIDR, /64, fc00::/7.
Covered by the onestick VM test (client SLAAC address, ping and DNS over
the ULA, RA captured on the wire), assertion cases and a unit test for
the /64 prefix helper.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>