feat: unify polygon type for CSG and rendering

feat: unify polygon type for CSG and rendering

Extend SolidPolygon to support N-vertex convex polygons, enabling
direct use in CSG operations without triangulation. Move CSG boolean
operations (union, subtract, intersect) from the standalone CSG class
to AbstractCompositeShape for in-place modifications with simpler API.

- SolidPolygon now handles arbitrary convex polygons via fan triangulation
- CSG operations work directly on SolidPolygon, eliminating CSGPolygon
- Add chainable setters to AbstractCompositeShape for fluent API
- Add non-mutating methods to Point2D/Point3D/Transform
- Rename TexturedPolygon to TexturedTriangle for consistency
- Fix vertex cloning, polygon validation, collinear point detection
- Use ThreadLocal fields for thread-safe rendering state