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>

tests/lib.nix

@@ -6,7 +6,25 @@ let
   inherit (r) ipv4;
   ip = ipv4.parse;
 
+  inherit (r) ipv6;
+
   failures = lib.runTests {
+    testPrefix64 = {
+      expr = map ipv6.prefix64 [
+        "fd2c:cdef:cde9::1/64"
+        "fd2c:cdef:cde9:1:2:3:4:5"
+        "fd00::1"
+        "fd12:3456:789a:bc::1/64"
+        "fd12::5:6"
+      ];
+      expected = [
+        "fd2c:cdef:cde9:0::/64"
+        "fd2c:cdef:cde9:1::/64"
+        "fd00:0:0:0::/64"
+        "fd12:3456:789a:bc::/64"
+        "fd12:0:0:0::/64"
+      ];
+    };
     testParse = {
       expr = ip "192.168.10.1";
       expected = 3232238081;