feat: add option contracts to overview, fix default dates, add roadmap items

- Move option contracts from daily results table to overview cards (constant throughout backtest)
- Fix default dates to March 2026 (2026-03-02 to 2026-03-25)
- Add BT-004 backlog item: candlestick chart with portfolio value line on secondary axis
- Add BT-005 backlog item: defer entry spot derivation to backtest run (not on every date change)

docs/roadmap/ROADMAP.yaml

@@ -13,6 +13,8 @@ notes:
   - Pre-alpha policy: we may cut or replace old features without backward compatibility until alpha is declared.
   - Alpha migration policy: once alpha is declared, compatibility only needs to move forward; backward migrations are not required.
 priority_queue:
+  - BT-005
+  - BT-004
   - EXEC-002
   - DATA-DB-005
   - DATA-002A
@@ -48,6 +50,8 @@ recently_completed:
   - CORE-002B
 states:
   backlog:
+    - BT-005
+    - BT-004
     - DATA-DB-005
     - DATA-DB-006
     - EXEC-002