Update implementation plan with camera follow feature

Add completed camera follow features to implementation plan:
- Camera follow for selected bodies
- Distance preservation when switching bodies
- Proper orbital rotation using camera.up

docs/implementation_plan.md

@@ -320,6 +320,8 @@ GUI-only application with interactive 3D visualization.
 - **Comprehensive test suite (8 test files, 39+ assertions)**
 - **Build system with automated testing**
 - **UI body selection and information display (raygui integration)**
+- **Camera follow feature for selected bodies**
+- **Camera follow improvements: distance preservation and proper orbital rotation**
 
 ### 🔨 Remaining/Future Work
 - More accurate integration methods (Newton-Raphson propagation)