refactor(demos): adopt new sixth-3d API and add tutorial docs

refactor(demos): adopt new sixth-3d API and add tutorial docs

Update all demos to use the modernized sixth-3d engine API with fluent
factory methods and in-place CSG operations. Add comprehensive tutorial
documentation in the Essentials section with screenshots and code examples.

- Use static imports: Point3D.point(), Color.hex()
- Replace CSG class usage with in-place union/subtract/intersect methods
- Update to renamed classes: SolidTriangle, TexturedTriangle
- Use SolidPolygon.quad() for quad surfaces instead of two triangles
- Add Minimal Example tutorial with step-by-step walkthrough
- Add screenshots for CSG, Coordinate System, Shape Gallery, Winding Order
- Rename AxisArrowsDemo to CoordinateSystemDemo with class references