Add a generic NAT table: port forward, source NAT, masquerade

router.firewall.nat replaces the dnat/toPort/hairpin fields of the filter
rules. Filter rules gain natOnly, and may use wan with log and rate limit
in the web UI. The Firewall page edits the NAT table and can add the
matching allow rule. Settings schema 1 (migrations/1.jq) converts old
forwards.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

tests/update.nix

@@ -50,6 +50,8 @@ let
     name = "v1";
     version = "1.0.0";
     serial = 1000;
+    # Settings version 0: the real migrations are not part of this test.
+    extra = "rm $out/migrations/*.jq";
   };
   v2 = mkVersion {
     name = "v2";