Function app_temp_dir 

pub fn app_temp_dir() -> PathBuf

An app-writable scratch directory. The OS temp dir is NOT app-writable on every target (Android reports getCacheDir()), so a backend records the right location at startup and this is how an app asks for it. For derived files an app can rebuild: rendered documents, thumbnails, export staging. An app-writable scratch directory: the backend-supplied one, else std::env::temp_dir(). Used by the file-save flow (docs/files.md) to stage bytes before the native save picker.