nixos/home: Use Digiweb static IP for river WAN

Digiweb assigned river a static IPv4 (84.203.124.128). Point river's
public address at it and have pppd actively request it as the IPCP local
address (local:remote form) rather than passively accepting whatever the
peer offers via noipdefault. The address stays sourced from the
routersPubV4 constant, so the ipsec l2 mesh peer, the nft DNS redirect,
and the cross-router route all follow.

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

lib/constants.nix

@@ -322,7 +322,7 @@ rec {
       "stream"
     ];
     routersPubV4 = [
-      "109.255.108.88"
+      "84.203.124.128" # river: Digiweb static
       "109.255.108.121"
     ];