refactor(demo): simplify benchmark teardown and improve CSG demo structure

refactor(demo): simplify benchmark teardown and improve CSG demo structure

- Replace manual shape removal loops with shapes.clear() in all benchmark
  teardown methods for cleaner and more efficient cleanup
- Extract createCube() and createSphere() factory methods in CSGDemo to
  encapsulate shape creation with appropriate shading settings (cubes with
  shading enabled, spheres with shading disabled for glow effect)
- Update CSG demo colors to green/orange palette and enhance lighting
  configuration with brighter lights and warm fill tint
- Use static color imports for cleaner code in description panels