leaderboard: log the local-fallback path instead of printing

leaderboard.py

@@ -319,7 +319,7 @@ def load_leaderboard_split() -> tuple[pd.DataFrame, pd.DataFrame]:
     """
     rows = _load_rows_from_hub()
     if rows is None:
-        print("[load_leaderboard_split] falling back to local results.jsonl")
+        logger.warning("Hub read failed; falling back to local results.jsonl")
         rows = _load_rows_from_local()
     if not rows:
         return (