docs: expand engine documentation and modularize SVG diagrams

docs: expand engine documentation and modularize SVG diagrams

Move all inline SVG diagrams into separate files for better
organization, reusability, and easier maintenance. Expand documentation
across core geometry classes (Point3D, Vertex), rendering pipeline,
lighting system, CSG operations, frustum culling, and texture mapping.

- Extract 21 SVG diagrams from org files into standalone .svg files
- Add comprehensive Color class docs explaining mutable field design
- Document Point3D/Vertex distinction with visual comparison diagram
- Expand geometry docs: edges, faces, meshes, normals, winding order
- Add documentation index table to AGENTS.md
- Update all diagram colors for dark theme compatibility
- Rename lerp to interpolate for clearer API naming