feat(BT-002): add historical snapshot provider

app/services/event_comparison_ui.py

@@ -135,6 +135,18 @@ class EventComparisonPageService:
         preset = self.event_preset_service.get_preset(preset_slug)
         return tuple(preset.scenario_overrides.default_template_slugs)
 
+    def derive_entry_spot(self, *, preset_slug: str, template_slugs: tuple[str, ...]) -> float:
+        if not template_slugs:
+            raise ValueError("Select at least one strategy template.")
+        scenario = self.comparison_service.preview_scenario_from_inputs(
+            preset_slug=preset_slug,
+            template_slugs=template_slugs,
+            underlying_units=1.0,
+            loan_amount=0.0,
+            margin_call_ltv=0.75,
+        )
+        return float(scenario.initial_portfolio.entry_spot)
+
     def preview_scenario(
         self,
         *,