Update avatar_layers implementation, assets and documentation

src/AvatarLayerImages.cs

@@ -0,0 +1,26 @@
+using System;
+using Cthangover.Core.Mods;
+using Cthangover.Core.Mods.Resolvers;
+using Godot;
+
+namespace Cthangover.Mods.AvatarLayers
+{
+    /// <summary>Loads exact PNG paths through mod providers, so ZIP assets and overrides work alike.</summary>
+    internal static class AvatarLayerImages
+    {
+        // The renderer owns the returned texture and must dispose it after detaching all node references.
+        internal static Texture2D Load(string path)
+        {
+            if (!path.EndsWith(".png", StringComparison.Ordinal))
+                throw new ArgumentException($"Avatar layer image '{path}' must be an exact .png path.");
+            if (!Files.Collect("avatar_layers").TryGetValue(path, out var file))
+                throw new ArgumentException($"Avatar layer image '{path}' was not found in avatar_layers.");
+            var bytes = ModManager.Instance.ReadFileBinary(file.ModId, file.FullPath);
+            if (bytes == null) throw new ArgumentException($"Cannot read avatar layer image '{path}'.");
+            using var image = new Image();
+            if (image.LoadPngFromBuffer(bytes) != Error.Ok)
+                throw new ArgumentException($"Avatar layer image '{path}' is not a valid PNG.");
+            return ImageTexture.CreateFromImage(image);
+        }
+    }
+}