sync: add cross-subsystem event bus for config consistency

Add EventBus with loop guards to keep firewall, dnsmasq, wireguard,
and network configs consistent. Handlers emit SyncEvent after mutations;
subscribers compute diffs and write JSON without manual cascade loops.

tests/test_firewall.py

@@ -615,16 +615,21 @@ class TestParseAllZonesOutput:
 
     def test_all_default_fields_present(self):
         result = firewall._parse_all_zones_output(
-            "dmz\n"
-            "  target: default\n"
-            "  interfaces: \n"
-            "  services: \n"
-            "  rich rules: \n"
+            "dmz\n  target: default\n  interfaces: \n  services: \n  rich rules: \n"
         )
         zone = result["dmz"]
         for field in (
-            "interfaces", "sources", "services", "ports", "protocols",
-            "forward-ports", "masquerade", "ics", "icmp-blocks", "module",
-            "target", "rich-rules",
+            "interfaces",
+            "sources",
+            "services",
+            "ports",
+            "protocols",
+            "forward-ports",
+            "masquerade",
+            "ics",
+            "icmp-blocks",
+            "module",
+            "target",
+            "rich-rules",
         ):
             assert field in zone, f"Missing field: {field}"