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/backtest_viewer.html

@@ -152,11 +152,15 @@ tr.below-threshold { color: #484f58; }
 </div>
 
 <script>
-let DATA = null;
-let currentIdx = 0;
-let playing = false;
-let playTimer = null;
-let rebalanceDays = [];
+// Use window. to allow external access (for Playwright screenshot injection)
+window.DATA = null;
+window.currentIdx = 0;
+window.playing = false;
+window.playTimer = null;
+window.rebalanceDays = [];
+
+// Convenience aliases for script-internal use
+const DATA_PROXY = { get data() { return window.DATA; }, set data(v) { window.DATA = v; } };
 
 const $ = id => document.getElementById(id);