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/math/aabb/getaxis.md

@@ -0,0 +1,23 @@
+# OBB::GetAxis
+
+```cpp
+Vector3 GetAxis(int index) const
+```
+
+获取 OBB 的局部坐标轴。
+
+**参数：**
+- `index` - 轴索引（0=X轴, 1=Y轴, 2=Z轴）
+
+**返回：** `Vector3` - 对应轴的方向向量
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+OBB obb = ...;
+Vector3 xAxis = obb.GetAxis(0);  // 局部 X 轴
+Vector3 yAxis = obb.GetAxis(1);  // 局部 Y 轴
+Vector3 zAxis = obb.GetAxis(2);  // 局部 Z 轴
+```