AtlasBuilder packs a directory tree of loose images into atlas pages plus
JSON metadata (deterministic shelf packing, incremental rebuilds, orphan
cleanup); TextureAtlas loads them back handing out Texture2DRegions, so
sprites from one page batch into a single draw call. The asset handle
generator maps .atlas files to TextureAtlas and skips page images.
Demonstrated in the sample (Assets/Atlases + 'atlas' console command),
wrapped as tools/MrGameEng.AtlasTool for build scripts.
Documented dependency exception: Atlases depends on Graphics and Assets.
