Add renderer development session notes

🤖 Generated with Claude Code - Co-Authored-By: Claude Sonnet 4.5

.gitignore

@@ -2,8 +2,8 @@
 build/
 orbit_sim
 
-# Session notes and conversation logs
-sessions/
+# Session notes and conversation logs (root level only)
+/sessions/
 
 # Editor/IDE files
 .vscode/

docs/sessions/2026-01-03-renderer-development.md

@@ -0,0 +1,46 @@
+# Renderer Development Session (2026-01-03)
+
+## Changes Made:
+
+### 1. Added orbit path visualization
+- Store orbital parameters (eccentricity, semi_major_axis) in CelestialBody struct
+- Render elliptical orbit paths for all non-root bodies
+- Orbits drawn as 100-segment line loops with semi-transparent body colors
+- Correctly places parent body at one focus of the ellipse
+
+### 2. Implemented sim-to-render coordinate transform
+- Simulation uses XY plane (natural for physics)
+- Renderer uses XZ plane (natural for 3D top-down view)
+- `sim_to_render()` applies 90° rotation around X-axis: Y→Z, Z→-Y
+- All positions and orbits use consistent transformation
+
+### 3. Fixed render scaling issue (renderer.cpp:26-27)
+**Problem**: size_scale (5e-7) and distance_scale (1e-9) had inconsistent ratios
+- Sun radius: 6.96e8 m × 5e-7 = 348 units (engulfed orbits!)
+- Earth orbit: 1.496e11 m × 1e-9 = 149.6 units
+
+**Solution**: Set size_scale = 1e-9 to match distance_scale
+- Result: Sun radius now 0.696 units, orbits clearly visible
+- Maintains physical accuracy with uniform scaling
+
+### 4. Visual improvements
+- Changed bodies to wireframe spheres (DrawSphereWires)
+- Custom dark grid (RGB 20,20,20) instead of default bright grid
+- Bodies use saturated colors for visibility
+- Minimum radius (0.5 units) ensures small bodies remain visible
+
+## Rendering Parameters
+
+- `distance_scale = 1e-9` (1 render unit = 1 billion meters = ~0.0067 AU)
+- `size_scale = 1e-9` (same as distance_scale for physical accuracy)
+- `min_radius = 0.5` (minimum visible body size)
+
+## Commits
+
+- `d7d49b7` Add orbit path visualization with sim-to-render coordinate transform
+- `9b4009d` Set DEFAULT_HEADLESS to 0 for renderer development
+- `4e2f00a` Fix render scaling to match simulation coordinates
+
+## Known Issues
+
+- No known rendering issues at this time