fix: address pre-push review findings

tests/test_backtest_data_quality.py

@@ -51,3 +51,11 @@ def test_live_and_backtest_outputs_publish_panel_and_coverage_metadata():
         "panel_count": 9,
     }
     assert metadata["staleness_days"] == backtest.METRIC_MAX_AGE_DAYS
+
+
+def test_chart_data_exposes_metric_values_under_frontend_contract():
+    result = backtest.run_backtest()
+    entries = [entry for entry in result["chart_data"] if entry.get("metric_values")]
+
+    assert entries
+    assert all("metrics" not in entry for entry in entries)