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.

lib/firewall.py

@@ -155,9 +155,7 @@ def _parse_all_zones_output(output: str) -> dict[str, dict[str, Any]]:
 
     # Finalize last zone
     if current_name is not None and current_lines:
-        zones[current_name] = _parse_zone_output(
-            current_name, "\n".join(current_lines)
-        )
+        zones[current_name] = _parse_zone_output(current_name, "\n".join(current_lines))
 
     return zones