lib/home: Centralise advertised search domains

radvd's DNSSL and kea's domain-search hardcoded the same list of
search domains in two places. Hoist it to lib.my.c.home.searchDomains
so there is a single source of truth (and so other consumers, such as
statically-configured boxes, can reuse it). No change to what is
advertised.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/constants.nix

@@ -307,6 +307,8 @@ rec {
 
   home = rec {
     domain = "h.${pubDomain}";
+    # Search domains advertised to clients (radvd DNSSL / kea domain-search)
+    searchDomains = [ domain "dyn.${domain}" colony.domain britway.domain ];
     vlans = {
       hi = 100;
       lo = 110;