refactor(pre-alpha): align preview and runtime fixture validation

tests/test_event_comparison.py

@@ -315,3 +315,43 @@ def test_event_comparison_uses_preset_defaults_only_when_template_slugs_are_omit
         "protective-put-90pct-12m",
         "ladder-50-50-atm-95pct-12m",
     ]
+
+    with pytest.raises(ValueError, match="at least one template slug"):
+        service.materialize_scenario(
+            service.event_preset_service.get_preset("gld-jan-2024-selloff"),
+            initial_portfolio=BacktestPortfolioState(
+                currency="USD",
+                underlying_units=1000.0,
+                entry_spot=100.0,
+                loan_amount=68_000.0,
+                margin_call_ltv=0.75,
+            ),
+            template_slugs=(),
+        )
+
+
+def test_event_comparison_materialize_scenario_rejects_explicit_empty_history() -> None:
+    provider = SyntheticHistoricalProvider(
+        source=FakeHistorySource(FIXTURE_HISTORY),
+        implied_volatility=0.35,
+        risk_free_rate=0.0,
+    )
+    service = EventComparisonService(
+        provider=provider,
+        template_service=StrategyTemplateService(),
+        event_preset_service=EventPresetService(repository=FileEventPresetRepository()),
+    )
+
+    with pytest.raises(ValueError, match="history must not be empty"):
+        service.materialize_scenario(
+            service.event_preset_service.get_preset("gld-jan-2024-selloff"),
+            initial_portfolio=BacktestPortfolioState(
+                currency="USD",
+                underlying_units=1000.0,
+                entry_spot=100.0,
+                loan_amount=68_000.0,
+                margin_call_ltv=0.75,
+            ),
+            template_slugs=("protective-put-atm-12m",),
+            history=[],
+        )