docs: fix README.md project structure to match actual codebase

Engine modules:
- Audio: remove AudioSource/Listener components (they belong in Components/), add IAudioEffect.h
- Components: add AudioSourceComponent.h and AudioListenerComponent.h
- Math: remove non-existent OBB.h
- Resources: fix DependencyGraph.h -> ResourceDependencyGraph.h
- Threading: add TaskSystemConfig.h
- RHI: remove incorrect RHIDescriptor.h (already covered by RHIDescriptorPool.h)

Backend fixes:
- OpenGL: add OpenGLScreenshot.h
- D3D12: fix file order (D3D12Enum.h before D3D12Types.h)

docs/api/resources/texture-import-settings/savetojson.md

@@ -0,0 +1,26 @@
+# TextureImportSettings::SaveToJSON
+
+```cpp
+Containers::String SaveToJSON() const override;
+```
+
+将当前设置序列化为 JSON 字符串。
+
+**返回：** `Containers::String` - JSON 格式的设置字符串
+
+**注意：** 当前为 stub 实现，始终返回空字符串。
+
+**线程安全：** ✅
+
+**示例：**
+
+```cpp
+TextureImportSettings settings;
+settings.SetSRGB(true);
+Containers::String json = settings.SaveToJSON();
+// 当前实现返回空字符串
+```
+
+## 相关文档
+
+- [TextureImportSettings 总览](textureimportsettings.md) - 返回类总览