feat(core): add SurfaceDecoder for A8R8G8B8 and BCn formats

src/AcDream.Core/Textures/DecodedTexture.cs

@@ -0,0 +1,10 @@
+namespace AcDream.Core.Textures;
+
+public sealed record DecodedTexture(byte[] Rgba8, int Width, int Height)
+{
+    /// <summary>1x1 magenta fallback for missing/unsupported textures.</summary>
+    public static readonly DecodedTexture Magenta = new(
+        Rgba8: [0xFF, 0x00, 0xFF, 0xFF],
+        Width: 1,
+        Height: 1);
+}