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/default.nix

@@ -11,12 +11,6 @@ in
 {
   nixos.systems."${name}" = {
     assignments = {
-      modem = {
-        ipv4 = {
-          address = net.cidr.host (254 - index) prefixes.modem.v4;
-          gateway = null;
-        };
-      };
       core = {
         name = "${name}-core";
         inherit domain;
@@ -100,9 +94,11 @@ in
 
     configuration = { lib, pkgs, config, assignments, allAssignments, ... }:
       let
-        inherit (lib) mkIf mkMerge mkForce;
-        inherit (lib.my) networkdAssignment;
+        inherit (lib) mkIf mkMerge mkForce optionalString concatStringsSep;
+        inherit (lib.my) mkOpt' networkdAssignment;
         inherit (lib.my.c) networkd;
+
+        cfg = config.my.homeRouter;
       in
       {
         imports = map (m: import m index) [
@@ -112,6 +108,20 @@ in
           ./kea.nix
         ];
 
+        # Per-box WAN-management specifics: the Virgin Media modem on stream lives on the `wan`
+        # interface itself, whereas river's ONT sits on its own interface. Declared as options the
+        # box sets so routing-common itself carries no modem/ONT knowledge.
+        options.my.homeRouter = with lib.types; {
+          dns.wanSkipBroadcasts = mkOpt' (listOf str) [ ] ''
+            Broadcast addresses to exclude when auto-selecting the router's own `wan` A record,
+            for extra static subnets that share the `wan` interface.
+          '';
+          firewall.untrustedRejectV4 = mkOpt' (listOf str) [ ] ''
+            IPv4 prefixes untrusted clients must be explicitly rejected from reaching. Only needed
+            for subnets sharing the `wan` interface, since `wan` egress is otherwise accepted.
+          '';
+        };
+
         config = {
           environment = {
             systemPackages = with pkgs; [
@@ -338,7 +348,7 @@ in
                     return
                   }
                   chain filter-untrusted {
-                    ip daddr ${prefixes.modem.v4} reject
+                    ${optionalString (cfg.firewall.untrustedRejectV4 != [ ]) "ip daddr { ${concatStringsSep ", " cfg.firewall.untrustedRejectV4} } reject"}
                     oifname wan accept
                     return
                   }

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;