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
emil28092005
2d30dec01c