docs(rendering): document render pipeline, lighting, and optimize normals

docs(rendering): document render pipeline, lighting, and optimize normals

Add comprehensive documentation for the rendering pipeline with SVG
diagrams showing the Shapes→Transform→Sort→Paint→Blit flow. Document
double-buffering, painter's algorithm, and frame listeners.

Add shading & lighting guide covering Lambert cosine law, light sources,
ambient light, and distance attenuation with visual diagrams.

Refactor Plane to expose computeNormal() for zero-allocation normal
computation, and move canvas clearing into parallel paint phase for
better memory bandwidth utilization.