Add MrGameEng.Atlases: texture atlas builder, runtime loader and CLI tool

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.

CLAUDE.md

@@ -19,11 +19,13 @@ docs/     architecture, conventions, roadmap (Russian)
 Engine modules: `Core` (game loop, ECS world, scenes, time), `Graphics` (custom batched
 renderer, camera, sprites), `Input`, `Audio`, `Assets` (runtime loading, no content
 pipeline), `Assets.Generator` (Roslyn source generator for typed asset handles),
+`Atlases` (texture-atlas builder + runtime loader; CLI wrapper in `tools/MrGameEng.AtlasTool`),
 `UI` (Myra integration: `scene.UseUI()` after `UseRenderer2D()`), `DevConsole`
 (in-game console capturing `Core.Log`; `scene.UseDevConsole()` last in OnLoad).
 Dependency rule: every module may depend only on `Core`; `Core` depends only on
 MonoGame and Friflo.Engine.ECS. `Assets.Generator` is a netstandard2.0 analyzer.
-Documented exception: Myra renders with its own SpriteBatch internally.
+Documented exceptions: Myra renders with its own SpriteBatch internally; `Atlases`
+depends on `Graphics` (Texture2DRegion) and `Assets` (loader registration).
 
 ## Commands