test: add local 2x2 route topology

src/archive_clients/syncthing.py

@@ -325,6 +325,13 @@ def _ack_name(nonce_owner: str, observer: str) -> str:
 
 def _atomic_json(path: Path, value: dict[str, Any]) -> None:
     encoded = json.dumps(value, sort_keys=True, separators=(",", ":"))
+    try:
+        if path.read_text(encoding="utf-8") == encoded:
+            return
+    except FileNotFoundError:
+        pass
+    except (OSError, UnicodeError) as exc:
+        raise RouteSetupError("route verification file is unreadable") from exc
     temporary = path.with_name(
         f".{path.name}.{os.getpid()}.{uuid.uuid4().hex}.tmp"
     )