nixos/home: De-share Virgin Media modem specifics from routing-common

The modem management subnet is stream-specific (it shares stream's `wan`
interface), so it no longer belongs in the shared routing-common. Drop the
`modem` network assignment and configure the address directly on stream's
`wan` (modemV4). The firewall reject and DNS wan-A-record skipBroadcasts that
existed only because of the modem move to per-box options
`my.homeRouter.{firewall.untrustedRejectV4,dns.wanSkipBroadcasts}`, set solely
by stream. river needs neither: its ONT sits on a separate interface already
blocked by the interface-based forward rules.

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

nixos/boxes/home/routing-common/dns.nix

@@ -206,7 +206,7 @@ in
             ${name} IN LUA ${lib.my.dns.ifaceA {
               inherit pkgs;
               iface = "wan";
-              skipBroadcasts = [ (lib.my.netBroadcast prefixes.modem.v4) ];
+              skipBroadcasts = config.my.homeRouter.dns.wanSkipBroadcasts;
             }}
             ${otherName} IN LUA ${lib.my.dns.lookupIP {
               inherit pkgs;