feat: add HTML report screenshot generation via Playwright

- Add html_report.py module for Playwright-based screenshot generation
- Add generate_html_report() method to SimpleRotationStrategy
- Modify backtest_viewer.html to use window-scoped variables for external injection
- Inject monthly/yearly returns table into screenshot
- Auto-generate HTML report in __main__ after export_results()

Output: simple_rotation_html_report.png with ranking table + monthly returns

rotation/simple_rotation.py

@@ -1418,6 +1418,37 @@ class SimpleRotationStrategy:
         plt.close()
         print(f"  + Report: {chart_path}")
 
+    def generate_html_report(self, output_dir: str = None, target_date: str = None):
+        """Generate HTML-based report screenshot using Playwright.
+
+        Renders backtest_viewer.html in headless Chromium and captures
+        the ranking table + monthly returns as a PNG.
+
+        Args:
+            output_dir: Directory to save the PNG (default: rotation/results)
+            target_date: Date to navigate to (YYYY-MM-DD). None = last day.
+        """
+        from rotation.html_report import generate_html_screenshot
+
+        if output_dir is None:
+            output_dir = Path(__file__).parent / 'results'
+        output_dir = Path(output_dir)
+        output_dir.mkdir(parents=True, exist_ok=True)
+
+        detail_path = output_dir / 'simple_rotation_detail.json'
+        if not detail_path.exists():
+            print(f"  x Detail JSON not found: {detail_path}")
+            print("    Run export_results() first.")
+            return
+
+        output_path = output_dir / 'simple_rotation_html_report.png'
+        generate_html_screenshot(
+            detail_json_path=str(detail_path),
+            output_path=str(output_path),
+            target_date=target_date,
+            daily_records=self.daily_records,
+        )
+
 
 # ============================================================
 # Entry point
@@ -1433,3 +1464,4 @@ if __name__ == "__main__":
     if result:
         strategy.export_results()
         strategy.generate_report()
+        strategy.generate_html_report()