fix(backtest): squeeze yfinance Close series to avoid DataFrame iteration error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backtest/backtest.py

@@ -35,8 +35,12 @@ def _fetch_prices(ticker: str, start: datetime, end: datetime) -> dict[str, floa
     if data.empty:
         return cached
 
+    close_series = data["Close"]
+    if hasattr(close_series, "squeeze"):
+        close_series = close_series.squeeze()
+
     fetched: dict[str, float] = {}
-    for ts, close_val in data["Close"].items():
+    for ts, close_val in close_series.items():
         date_key = ts.to_pydatetime().replace(tzinfo=None).strftime("%Y-%m-%d")
         fetched[date_key] = float(close_val)