feat: Dear ImGui editor, GLTF materials, scene serialization

- ImGuiLayer: entity inspector (Transform/Material/Light/Camera editing),
  hierarchy panel, debug overlay with FPS graph (rlImgui-cs + ImGui.NET)
- GltfLoader.LoadWithMaterials: extracts PBR albedo, roughness, metallic,
  base color texture from glTF files
- SceneSerializer: save/load ECS world to/from JSON with custom
  Vector3/Quaternion converters, 6 tests
- ImGui disabled in tour/test mode to keep screenshots clean
- 66/66 tests passing

imgui.ini

@@ -0,0 +1,20 @@
+[Window][Debug##Default]
+Pos=60,60
+Size=400,400
+Collapsed=0
+
+[Window][Debug]
+Pos=10,10
+Size=216,107
+Collapsed=0
+
+[Window][Hierarchy]
+Pos=29,195
+Size=250,398
+Collapsed=0
+
+[Window][Inspector]
+Pos=964,309
+Size=300,400
+Collapsed=0
+