feat(demos): add CSG and axis demos, reorganize into essentials subpackage

feat(demos): add CSG and axis demos, reorganize into essentials subpackage

Add two new demonstration apps:
- CSGDemo: showcases boolean operations (subtract, union, intersect) on
  3D shapes using the BSP-based CSG system from sixth-3d
- AxisArrowsDemo: displays coordinate system axes with colored arrows
  (X=red, Y=green, Z=blue) and text labels

Reorganize demo structure:
- Move basic demos (MinimalExample, WindingOrderDemo, ShapeGalleryDemo)
  to examples.essentials subpackage for cleaner organization
- Rename ShadedShapesDemo to ShapeGalleryDemo for clarity
- Remove RandomPolygonsDemo (redundant with other polygon demos)

Update launcher with two-tier layout: "Essentials" section for core
demos, "Demos" section for advanced applications. Simplify arrow
constructors in ArrowDemo and ShapeGalleryDemo using auto-calculated
tip dimensions.