Add UI implementation plan for body selection feature

docs/ui_implementation_plan.md

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+# UI Implementation Plan - Body Selection and Information Display
+
+## Overview
+Add UI elements using raygui to list celestial bodies and display detailed information about selected bodies.
+
+## UI Library Choice
+**raygui** - raylib's official immediate-mode GUI library
+- Seamless integration with existing raylib code
+- Lightweight and follows raylib's C-style conventions
+- Provides `GuiListView()` for body lists and `GuiWindowBox()` for information panels
+- No additional dependencies beyond raylib
+
+## Implementation Phases
+
+### Phase 1: Add raygui Submodule
+1. **Add raygui as git submodule**
+   ```bash
+   git submodule add https://github.com/raysan5/raygui.git ext/raygui
+   ```
+   - Follows existing pattern with `ext/` directory
+   - Keeps raygui version-controlled and easily updatable
+   - Matches current dependency management approach
+
+2. **Update .gitmodules** (automatic)
+   - Will add new entry for raygui submodule
+
+### Phase 2: Update Build System
+1. **Modify Makefile**
+   - Add raygui include path: `-I./ext/raygui/src`
+   - Add raygui.c compilation rule (similar to tomlc17)
+   - Update linking to include raygui objects
+
+2. **Integration approach**
+   - Follow existing pattern from tomlc17 integration
+   - Add raygui.c to C_SOURCES or create separate raygui object rule
+   - Ensure raygui is built before main target
+
+### Phase 3: Code Integration
+1. **Update renderer.h**
+   ```cpp
+   #include "raygui.h"  // after raylib include
+   
+   struct RenderState {
+       // existing fields...
+       int selected_body_index;    // -1 = no selection
+       bool show_body_list;        // Toggle for list panel
+       bool show_body_info;        // Toggle for info panel
+   };
+   ```
+
+2. **Add UI functions to renderer.h**
+   ```cpp
+   // UI rendering functions
+   void render_body_list_ui(SimulationState* sim, RenderState* render_state);
+   void render_body_info_ui(SimulationState* sim, RenderState* render_state);
+   ```
+
+### Phase 4: Implementation Details
+1. **UI Layout Strategy**
+   - Left panel (200px): Body list using `GuiListView()`
+   - Right panel (250px): Detailed info using `GuiLabel()` and text
+   - Both panels toggleable with keyboard shortcuts
+
+2. **raygui-specific considerations**
+   - raygui uses immediate-mode, no state management needed
+   - Requires `#define RAYGUI_IMPLEMENTATION` in one .c file
+   - Can be compiled alongside raylib without conflicts
+
+3. **Controls Integration**
+   - 'B' key: Toggle body list
+   - 'I' key: Existing info toggle (extend to show body info)
+   - Mouse: Select from list, drag windows
+   - ESC: Close panels (or keep for quit)
+
+4. **Body Information Display**
+   When a body is selected, display:
+   - Name, mass, radius
+   - Current position/velocity
+   - Orbital parameters (eccentricity, semi-major axis)
+   - Parent body and SOI radius
+   - Real-time orbital metrics
+
+### Phase 5: Integration and Polish
+1. **Update main render loop**
+   - Call UI rendering functions after 3D rendering but before `EndDrawing()`
+   - Maintain proper depth ordering
+
+2. **Enhance user experience**
+   - Highlight selected body in 3D view
+   - Add visual indicators for selected body's orbit
+   - Camera focus option on selected body
+
+## Key Technical Details
+
+### raygui Functions to Use:
+- `GuiWindowBox(bounds, title)` - Draggable window containers
+- `GuiListView(bounds, text, scrollIndex, active)` - Scrollable body list
+- `GuiLabel(bounds, text)` - Information display
+- `GuiToggle(bounds, text, active)` - Toggle switches for panels
+
+### Data Flow:
+1. Simulation populates `SimulationState` with bodies
+2. UI reads body names from `sim->bodies[i].name`
+3. Selection updates `render_state->selected_body_index`
+4. Info panel displays detailed data from selected body
+
+### Layout Strategy:
+- Body list: Left side of screen (200px wide)
+- Info panel: Right side of screen (250px wide)
+- Both panels can be toggled independently
+- Maintain existing 3D viewport in center
+
+## Files to Modify:
+1. **Makefile** - Add raygui compilation
+2. **src/renderer.h** - Add UI declarations and RenderState extensions
+3. **src/renderer.cpp** - Implement UI rendering functions
+4. **src/main.cpp** - Initialize UI state and add keyboard controls
+
+## Integration Notes:
+- Maintains C-style architecture
+- Immediate-mode UI fits existing rendering loop
+- Minimal state management required
+- Follows existing dependency patterns (git submodules)