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/AtlasMetadata.cs

@@ -0,0 +1,83 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace MrGameEng.Atlases;
+
+/// <summary>
+/// Serializable description of one packed atlas: its page image files and the source-relative
+/// region keys with their pixel rectangles. Stored as a JSON <c>.atlas</c> file next to the pages.
+/// </summary>
+public sealed class AtlasMetadata
+{
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+    };
+
+    /// <summary>Format version, bumped on breaking metadata changes.</summary>
+    public int Version { get; init; } = 1;
+
+    /// <summary>Atlas name (group key with '/' replaced by '.').</summary>
+    public string Name { get; init; } = "";
+
+    /// <summary>Maximum page size the atlas was built with (staleness check input).</summary>
+    public int PageSize { get; init; }
+
+    /// <summary>Padding in pixels the atlas was built with (staleness check input).</summary>
+    public int Padding { get; init; }
+
+    /// <summary>Page image files (relative to the metadata file), in page-index order.</summary>
+    public List<AtlasPage> Pages { get; init; } = [];
+
+    /// <summary>Packed regions, sorted by key.</summary>
+    public List<AtlasRegion> Regions { get; init; } = [];
+
+    /// <summary>Serializes this metadata to indented JSON.</summary>
+    public string ToJson() => JsonSerializer.Serialize(this, JsonOptions);
+
+    /// <summary>Parses metadata from JSON produced by <see cref="ToJson"/>.</summary>
+    public static AtlasMetadata FromJson(string json) =>
+        JsonSerializer.Deserialize<AtlasMetadata>(json, JsonOptions)
+        ?? throw new InvalidDataException("Atlas metadata JSON deserialized to null.");
+}
+
+/// <summary>One page image of an atlas.</summary>
+public sealed class AtlasPage
+{
+    /// <summary>Image file name, relative to the metadata file.</summary>
+    public string File { get; init; } = "";
+
+    /// <summary>Page width in pixels.</summary>
+    public int Width { get; init; }
+
+    /// <summary>Page height in pixels.</summary>
+    public int Height { get; init; }
+}
+
+/// <summary>One packed source texture inside an atlas.</summary>
+public sealed class AtlasRegion
+{
+    /// <summary>
+    /// Region key: the source path relative to the build source root, forward slashes,
+    /// without the file extension (e.g. <c>Things/Pawn/Animal/Fox</c>).
+    /// </summary>
+    public string Key { get; init; } = "";
+
+    /// <summary>Index of the page containing this region.</summary>
+    public int Page { get; init; }
+
+    /// <summary>X of the region in page pixels.</summary>
+    public int X { get; init; }
+
+    /// <summary>Y of the region in page pixels.</summary>
+    public int Y { get; init; }
+
+    /// <summary>Region width in pixels.</summary>
+    [JsonPropertyName("w")]
+    public int Width { get; init; }
+
+    /// <summary>Region height in pixels.</summary>
+    [JsonPropertyName("h")]
+    public int Height { get; init; }
+}