feat(demos): add RGB cube demo, light source demo, and extended benchmark

feat(demos): add RGB cube demo, light source demo, and extended benchmark

Add RGBCubeDemo: volumetric 10x10x10 color cube with textured triangles
and per-cell color mapping (X=Red, Y=Green, Z=Blue).

Add LightSourceDemo: white sphere illuminated by two colored lights
(yellow + blue) demonstrating multiple light contribution and shading.

Extend GraphicsBenchmark with short/extended test modes. Extended mode
measures thread scaling from 1 to available cores with a 1.5x step
factor, outputting a matrix of FPS per test x thread count.

Update AGENTS.md to reflect current code organization. Remove
ArrowDemo.java and doc/overview.png. Add screenshots for lights and
RGB cube. Move inline TODOs from index.org to sixth-3d/TODO.org.

Update TexturedCubesTest to use TextureGenerator.glowingBorder instead
of inline AWT texture creation.