feat(BT-003B): add event comparison drilldown

tests/test_event_comparison_ui.py

@@ -199,3 +199,61 @@ def test_event_comparison_page_service_builds_chart_model_with_unhedged_referenc
     assert chart_model.series[0].name == "Unhedged collateral baseline"
     assert chart_model.series[1].name == "Protective Put ATM"
     assert len(chart_model.series[0].values) == len(chart_model.dates)
+
+
+def test_event_comparison_page_service_builds_drilldown_for_selected_ranking() -> None:
+    service = EventComparisonPageService()
+
+    report = service.run_read_only_comparison(
+        preset_slug="gld-jan-2024-selloff",
+        template_slugs=("protective-put-atm-12m", "protective-put-95pct-12m"),
+        underlying_units=1000.0,
+        loan_amount=68000.0,
+        margin_call_ltv=0.75,
+    )
+    drilldown = service.drilldown_model(report, template_slug="protective-put-95pct-12m")
+
+    assert drilldown.rank == 2
+    assert drilldown.template_name == "Protective Put 95%"
+    assert drilldown.margin_call_days_hedged == report.rankings[1].margin_call_days_hedged
+    assert drilldown.hedge_cost == report.rankings[1].hedge_cost
+    assert drilldown.final_equity == report.rankings[1].final_equity
+    assert (
+        drilldown.total_option_payoff_realized == report.rankings[1].result.summary_metrics.total_option_payoff_realized
+    )
+    assert drilldown.worst_ltv_date == "2024-01-08"
+    assert drilldown.rows[0].date == "2024-01-02"
+    assert drilldown.rows[-1].date == "2024-01-08"
+
+
+def test_event_comparison_page_service_defaults_drilldown_to_top_ranked_strategy() -> None:
+    service = EventComparisonPageService()
+
+    report = service.run_read_only_comparison(
+        preset_slug="gld-jan-2024-selloff",
+        template_slugs=("protective-put-atm-12m", "protective-put-95pct-12m"),
+        underlying_units=1000.0,
+        loan_amount=68000.0,
+        margin_call_ltv=0.75,
+    )
+
+    drilldown = service.drilldown_model(report)
+
+    assert drilldown.rank == 1
+    assert drilldown.template_slug == report.rankings[0].template_slug
+    assert drilldown.template_name == report.rankings[0].template_name
+
+
+def test_event_comparison_page_service_rejects_unknown_drilldown_template_slug() -> None:
+    service = EventComparisonPageService()
+
+    report = service.run_read_only_comparison(
+        preset_slug="gld-jan-2024-selloff",
+        template_slugs=("protective-put-atm-12m",),
+        underlying_units=1000.0,
+        loan_amount=68000.0,
+        margin_call_ltv=0.75,
+    )
+
+    with pytest.raises(ValueError, match="Unknown ranked template"):
+        service.drilldown_model(report, template_slug="missing-template")