feat: defer entry spot derivation to backtest run (BT-005)

- Remove async refresh_workspace_seeded_units from date change handlers
- Date changes now only call on_form_change() (updates cost estimates, marks results stale)
- Entry spot is derived only when user clicks Run button
- Form remains responsive during configuration
- No more API errors when changing dates while configuring other fields

docs/roadmap/ROADMAP.yaml

@@ -13,7 +13,6 @@ 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
@@ -25,6 +24,7 @@ priority_queue:
   - GCF-001
   - DATA-DB-006
 recently_completed:
+  - BT-005
   - CORE-003
   - CONV-001
   - DATA-DB-004
@@ -50,7 +50,6 @@ recently_completed:
   - CORE-002B
 states:
   backlog:
-    - BT-005
     - BT-004
     - DATA-DB-005
     - DATA-DB-006
@@ -63,6 +62,7 @@ states:
     - GCF-001
   in_progress: []
   done:
+    - BT-005
     - CORE-003
     - CONV-001
     - DATA-DB-003