Fix backtest summary.txt UnicodeEncodeError on Windows

write_text() defaults to the platform encoding (cp1252 on Windows),
which can't encode the arrow character used in the summary header.
Force UTF-8 explicitly so backtests complete on Windows hosts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backtest.py

@@ -694,7 +694,7 @@ def main() -> int:
     df.to_csv(out_dir / "trades.csv", index=False)
 
     summary = summarize(df)
-    (out_dir / "summary.txt").write_text(summary)
+    (out_dir / "summary.txt").write_text(summary, encoding="utf-8")
 
     print("\n" + summary)
     print(f"\n📁 trades.csv  → {out_dir/'trades.csv'}")