feat(csg): add constructive solid geometry with BSP tree boolean operations

feat(csg): add constructive solid geometry with BSP tree boolean operations

Add a complete CSG system that enables boolean operations (union, subtract,
intersect) on 3D solids using BSP trees. The implementation includes:

- CSG class with union/subtract/intersect/inverse operations
- CSGNode BSP tree for spatial partitioning and polygon clipping
- CSGPlane for polygon classification and splitting
- CSGPolygon N-gon type for BSP operations
- PolygonType enum for vertex classification

Integration with existing shapes via:
- extractSolidPolygons() method on AbstractCompositeShape
- SolidPolygonMesh composite for rendering CSG results
- CSG.fromCompositeShape() factory for converting existing primitives

Also includes supporting refactorings: rename coordinates field to vertices
for consistency, simplified constructors for arrow shapes with auto-calculated
tip dimensions, and Point3D/Vertex enhancements for vector math.