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.

tests/MrGameEng.Assets.Generator.Tests/AssetHandlesGeneratorTests.cs

@@ -86,6 +86,22 @@ public class AssetHandlesGeneratorTests
         Assert.DoesNotContain("Image", source);
     }
 
+    [Fact]
+    public void AtlasFiles_GetTextureAtlasHandles_AndPagesAreExcluded()
+    {
+        var source = RunGenerator(
+        [
+            @"D:\game\Assets\Atlases\Things.Pawn.atlas",
+            @"D:\game\Assets\Atlases\Things.Pawn.atlas.0.png",
+            @"D:\game\Assets\Atlases\Things.Pawn.atlas.1.png",
+        ]);
+
+        Assert.Contains(
+            "AssetRef<global::MrGameEng.Atlases.TextureAtlas> ThingsPawn = new(\"Atlases/Things.Pawn.atlas\")",
+            source);
+        Assert.DoesNotContain("Texture2D> ThingsPawn", source);
+    }
+
     [Fact]
     public void NestedDirectories_BecomeNestedClasses()
     {