fix(CORE-002C): explain undercollateralized historical seeds

tests/test_backtest_ui.py

@@ -131,6 +131,18 @@ def test_backtest_page_service_keeps_fixture_history_while_using_caller_portfoli
             },
             "BT-001A backtests are currently limited to GLD on this page",
         ),
+        (
+            {
+                "symbol": "GLD",
+                "start_date": date(2024, 1, 2),
+                "end_date": date(2024, 1, 8),
+                "template_slug": "protective-put-atm-12m",
+                "underlying_units": 1000.0,
+                "loan_amount": 145000.0,
+                "margin_call_ltv": 0.75,
+            },
+            "Historical scenario starts undercollateralized",
+        ),
     ],
 )
 def test_backtest_page_service_validation_errors_are_user_facing(kwargs: dict[str, object], message: str) -> None: