docs: Document stub/not-implemented methods in resources module

Fixed discrepancies between source code and documentation:
- AsyncLoader: Document Initialize() ignores workerThreadCount, Submit() doesn't do actual async loading, Update() is stub
- ResourceManager: Document UnloadUnused() and ReloadResource() are stubs
- ResourceCache: Document OnZeroRefCount() and Flush() are stubs
- ResourceDependencyGraph: Document TopologicalSort() returns empty (stub)
- ResourceFileSystem: Document GetResourceInfo() doesn't fill modifiedTime, EnumerateResources() is stub
- FileArchive: Document Enumerate() is stub
- ResourcePackageBuilder: Document AddDirectory() is stub
- ImportSettings: Document LoadFromJSON/SaveToJSON are stubs
- TextureImportSettings/MeshImportSettings: Document JSON methods are stubs
- TextureLoader/MeshLoader/MaterialLoader/ShaderLoader/AudioLoader: Document GetDefaultSettings() returns nullptr
- AudioLoader: Document ParseWAVData() is stub, Load() doesn't parse WAV headers
- ShaderLoader: Document DetectShaderType/ParseShaderSource are stubs
- MaterialLoader: Document ParseMaterialData() is stub
- Texture: Document Create() mipLevels=0 behavior, GenerateMipmaps() returns false
- Mesh: Document MeshLoader::Load() is example only
- IResourceLoader: Document GetDefaultSettings() returns nullptr for all loaders

docs/api/resources/resourcepackage/resourcepackage.md

@@ -47,13 +47,17 @@
 | 方法 | 描述 |
 |------|------|
 | `bool AddFile(const Containers::String& sourcePath, const Containers::String& relativePath)` | 添加单个文件到包 |
-| `bool AddDirectory(const Containers::String& sourceDir, const Containers::String& relativeBase = "")` | 添加整个目录到包 |
+| `bool AddDirectory(const Containers::String& sourceDir, const Containers::String& relativeBase = "")` | 添加整个目录到包（当前为 stub，始终返回 true） |
 | `void SetOutputPath(const Containers::String& path)` | 设置输出包文件路径 |
 | `const Containers::String& GetOutputPath() const` | 获取输出路径 |
 | `bool Build()` | 构建包文件 |
 | `float GetProgress() const` | 获取构建进度（0.0f ~ 1.0f） |
 | `const Containers::String& GetError() const` | 获取错误信息 |
 
+## 实现说明
+
+**注意**: `ResourcePackageBuilder::AddDirectory()` 当前为 stub，始终返回 true。
+
 ## 使用示例（构建）
 
 ```cpp