Function app_temp_dir 

pub fn app_temp_dir() -> PathBuf

An app-writable scratch directory (docs/files.md) — re-exported from day_spec::present so day_core::app_temp_dir() keeps working for the pieces layer’s file-save 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.