Implements a sprite rendering system for 2D billboards in 4D space: - SpriteSheet: Atlas metadata with frame UV calculation - Sprite: 4D-positioned billboard with builder pattern API - SpriteBatch: Collects sprites, provides back-to-front sorting - WFadeConfig: Configurable fade based on W distance from camera Key features: - Sprites always face camera in XYZ (classic billboard) - W-fade creates smooth transparency as sprites move through 4D - Linear W-fade: alpha = 1.0 at camera W, 0.0 at fade range - Sorting by 3D distance (W ignored since billboards project to XYZ) Includes 22 unit tests for W-fade math, sorting, and batch operations.