M4.36: Background Save and Export write a snapshot off the UI thread with status bar progress

- With Preferences > File Handling > Enable Background Save / Export on, File >
  Save, Save As, Save a Copy and exports encode and write a snapshot of the
  document on a worker thread; the call returns at once ({path, background}).
- The status bar shows a spinner and what is being written; ui.inspect lists
  the running jobs under `background`.
- The document takes the path and counts as saved as of the snapshot once the
  file is written: edits made meanwhile keep it modified.
- Quit and Close wait for running saves; the Save Changes dialog saves at once.
- The host gives a thread-safe writer (Services::write_shared, atomic on the
  desktop); without one (web, tests) everything runs at once as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/fileio/mod.rs

@@ -41,7 +41,9 @@ pub(crate) use load::source;
 pub use load::{Loaded, RasterImage, detect, file_name, load, load_with, open_bytes, open_bytes_with, open_template, raster_image};
 pub use native::with_compression_pref;
 pub use pdfimport::{LoadOptions, page_document};
-pub use save::{SAVE_FORMATS, SaveMode, SavePlan, save_filters, save_format, save_plan, save_with, stamp_save_dates, templates_folder};
+pub use save::{
+    SAVE_FORMATS, SaveJob, SaveMode, SavePlan, save_filters, save_format, save_job, save_plan, save_with, stamp_save_dates, templates_folder,
+};
 pub use svg::options_map as svg_options;
 /// Atomic file writes (a temporary file renamed over the target: a failed write never damages the
 /// file it replaces), for the apps' own writers too.