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.

src/MrGameEng.Atlases/MrGameEng.Atlases.csproj

@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="MrGameEng.Atlases.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="StbImageSharp" />
+    <PackageReference Include="StbImageWriteSharp" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MrGameEng.Core\MrGameEng.Core.csproj" />
+    <ProjectReference Include="..\MrGameEng.Graphics\MrGameEng.Graphics.csproj" />
+    <ProjectReference Include="..\MrGameEng.Assets\MrGameEng.Assets.csproj" />
+  </ItemGroup>
+
+</Project>