Add a static ULA per LAN, announced via SLAAC and RDNSS

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>

modules/router/options.nix

@@ -209,6 +209,19 @@ in
                 '';
               };
 
+              ipv6.address = mkOption {
+                type = types.nullOr types.str;
+                default = null;
+                example = "fd2c:cdef:cde9::1/64";
+                description = ''
+                  Static unique-local (ULA) address of the router on this LAN,
+                  as a /64. It is announced via SLAAC, so hosts get a stable
+                  internal IPv6 address even without a delegated prefix, and
+                  the router's resolver answers on it (also advertised as
+                  RDNSS). Independent of `ipv6.pd`.
+                '';
+              };
+
               allowManagement = mkOption {
                 type = types.bool;
                 default = true;