refactor(pipeline): MP1a - lift TextureKey out of the GL atlas class

src/AcDream.Content/TextureKey.cs

@@ -0,0 +1,30 @@
+using DatReaderWriter.Enums;
+using System;
+
+namespace AcDream.Content;
+
+// MP1a (2026-07-05): lifted verbatim out of TextureAtlasManager (GL class)
+// so the GL-free mesh-extraction path (MeshExtractor / ObjectMeshData) can
+// reference the atlas dedup key without a Silk.NET dependency. Field set,
+// equality semantics, and hash behavior are UNCHANGED.
+public struct TextureKey : IEquatable<TextureKey> {
+    public uint SurfaceId;
+    public uint PaletteId;
+    public StipplingType Stippling;
+    public bool IsSolid;
+
+    public bool Equals(TextureKey other) {
+        return SurfaceId == other.SurfaceId &&
+               PaletteId == other.PaletteId &&
+               Stippling == other.Stippling &&
+               IsSolid == other.IsSolid;
+    }
+
+    public override bool Equals(object? obj) {
+        return obj is TextureKey other && Equals(other);
+    }
+
+    public override int GetHashCode() {
+        return HashCode.Combine(SurfaceId, PaletteId, Stippling, IsSolid);
+    }
+}