Add comprehensive UI implementation summary

Document complete body selection feature implementation:
- Technical implementation details
- Code changes summary
- Usage instructions
- Future enhancement possibilities

docs/ui_implementation_summary.md

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+# UI Implementation Complete - Body Selection Feature
+
+## Summary
+Successfully implemented body selection UI functionality using raygui for the orbital mechanics simulation.
+
+## Completed Features
+
+### ✅ Phase 1: raygui Integration
+- **Added raygui as git submodule** in `ext/raygui/`
+- **Updated build system** to include raygui headers
+- **Integrated raygui implementation** in renderer.cpp
+
+### ✅ Phase 2: UI State Management
+- **Extended RenderState** with UI fields:
+  - `selected_body_index`: Track selected body (-1 = none)
+  - `show_body_list`: Toggle body list panel
+  - `show_body_info`: Toggle body info panel
+- **Initialized UI state** in main.cpp
+
+### ✅ Phase 3: Body List UI Panel
+- **Implemented `render_body_list_ui()`**:
+  - Left-side panel (200px x 400px)
+  - Scrollable list of all celestial bodies
+  - Uses `GuiWindowBox()` for draggable container
+  - Uses `GuiListView()` for body selection
+  - Updates selected_body_index on click
+  - Auto-opens info panel when body selected
+
+### ✅ Phase 4: Body Information Panel
+- **Implemented `render_body_info_ui()`**:
+  - Right-side panel (250px x 300px)
+  - Displays comprehensive body information:
+    - Name, mass, radius
+    - Current position and velocity
+    - Orbital elements (eccentricity, semi-major axis)
+    - Parent body and SOI radius
+  - Uses `GuiLabel()` for multi-line text display
+
+### ✅ Phase 5: User Controls
+- **Added 'B' key** to toggle body list panel
+- **Updated help text** to include new controls
+- **Maintained existing controls** (I for info, arrows for camera, etc.)
+
+### ✅ Phase 6: Integration
+- **Integrated UI rendering** into main render loop
+- **Proper depth ordering** (3D scene → UI panels → end frame)
+- **Memory management** for dynamic body list creation
+
+## Technical Implementation Details
+
+### raygui Functions Used:
+- `GuiWindowBox()` - Draggable window containers
+- `GuiListView()` - Scrollable body selection list
+- `GuiLabel()` - Multi-line information display
+
+### Body List Format:
+- Converts body names array to semicolon-separated string
+- Compatible with raygui's expected input format
+- Dynamic memory allocation based on body count
+
+### UI Layout:
+- **Body List Panel**: Left side, 200px wide, 400px tall
+- **Info Panel**: Right side, 250px wide, 300px tall
+- **Responsive positioning**: Info panel positioned from screen edge
+
+### State Management:
+- Selection state persisted across frames
+- Panel toggles independent of each other
+- Info panel auto-opens when body selected
+
+## Code Changes Summary
+
+### Files Modified:
+1. **Makefile** - Added raygui include paths
+2. **src/renderer.h** - Added UI declarations and RenderState extensions
+3. **src/renderer.cpp** - Implemented UI functions and raygui integration
+4. **src/main.cpp** - Added UI state initialization and control help
+5. **.gitmodules** - Added raygui submodule entry
+
+### New Functions:
+- `render_body_list_ui()` - Body list panel rendering
+- `render_body_info_ui()` - Body information panel rendering
+
+### Controls Added:
+- **B key**: Toggle body list panel
+- **Mouse click**: Select body from list
+- **Window X**: Close individual panels
+
+## Testing and Verification
+
+### Build Status:
+- ✅ Compiles successfully with raygui integration
+- ✅ No linking errors or missing dependencies
+- ✅ Maintains existing functionality
+
+### Code Quality:
+- ✅ Follows existing C-style conventions
+- ✅ Proper memory management
+- ✅ Error handling for allocation failures
+- ✅ Consistent with project architecture
+
+## Usage Instructions
+
+1. **Launch simulation**: `./orbit_sim`
+2. **Open body list**: Press 'B' key
+3. **Select body**: Click on any body in the list
+4. **View information**: Info panel opens automatically
+5. **Close panels**: Click X button or press 'B' again
+
+## Future Enhancements (Optional)
+
+### Potential Improvements:
+- **Camera focus**: Option to focus camera on selected body
+- **Visual highlighting**: Highlight selected body in 3D view
+- **Enhanced info**: Add real-time orbital metrics
+- **Search functionality**: Filter body list by name
+- **Multiple selection**: Select multiple bodies for comparison
+
+### Advanced Features:
+- **Reference frame switching**: View from selected body's perspective
+- **Orbit manipulation**: Edit orbital parameters via UI
+- **Time controls**: Jump to specific orbital positions
+- **Export functionality**: Save body data to file
+
+## Conclusion
+
+The body selection UI feature has been successfully implemented with the following achievements:
+
+- **Complete UI functionality** with body list and information panels
+- **Seamless raygui integration** following project conventions
+- **User-friendly controls** with intuitive keyboard/mouse interaction
+- **Robust implementation** with proper memory management
+- **Extensible architecture** for future UI enhancements
+
+The implementation maintains the project's C-style architecture while adding modern UI capabilities, significantly improving the user experience for exploring and analyzing celestial body information in the orbital mechanics simulation.