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>

docs/control-protocol.md

@@ -8,7 +8,7 @@
 | `engine.execute` | `{command, params}` | run any engine or UI command (see `engine.commands`) |
 | `engine.commands` | | every command with label, shortcut, params doc, enablement |
 | `document.inspect` | | layer tree, selection, history, paint defaults |
-| `ui.inspect` | | tool, UI state, view, canvas rect, window size, perf |
+| `ui.inspect` | | tool, UI state, view, canvas rect, window size, perf, background saves and exports still running |
 | `ui.menu.list` / `ui.menu.invoke` | `{command, params}` | the full menu tree / invoke an item |
 | `ui.tool.select` / `ui.tool.list` | `{tool}` | |
 | `ui.pointer` | `{events:[{kind: down|drag|up|move|doubleclick, x, y, space?: "doc"|"screen", mods?}]}` | drive the active tool exactly like the mouse |
@@ -30,6 +30,7 @@
 | `app.export` PNG-8 and GIF | `{path?, format: png8 \| gif, colors?: 2–256, reduction?: perceptual \| selective \| adaptive \| web \| blackWhite \| gray, dither?: none \| diffusion \| pattern \| noise, ditherAmount?: 0–100, transparency?, matte?, interlaced?, …raster options}` | a palette of at most `colors` entries (art with that many colours or fewer keeps them exactly); pixels under half opacity become one transparent entry unless `transparency` is false, partly transparent edges are blended over `matte` (`none` keeps their colour). `png8` writes an indexed `.png` (1–8 bits per pixel); Export for Screens rows take both. The `png8Options` / `gifOptions` dialogs have these fields |
 | `app.export` Text and WebP | `{path?, format: txt, encoding?: utf8 \| utf16, lineEndings?: lf \| crlf, selectionOnly?}` / `{format: webp, lossless?, quality?}` | Text writes the stories in stacking order (back to front; a thread once; hidden objects and template layers left out), each line ended by `lineEndings`; `utf16` is little-endian with a byte order mark. Export As → Text opens `txtOptions` (fields `path`, `encoding`, `lineEndings`, `selectionOnly`; OK writes the file). WebP is always lossless: `lossless: false` adds a warning |
 | `engine.execute` `document.exportForOffice` | `{}` / `{path?, ppi?: 72 \| 150 \| 300, transparent?, artboard?}` | File → Save for Office Documents…: without params opens `saveForOffice` (fields `ppi`, `transparent`, `artboard`; OK picks the file); with params writes the artboard as a PNG (`path`, else a picked file) → `{path}`. Headless it returns `{dataBase64, width, height}` |
+| Background Save / Export | `app.save`, `app.export {path}`, `file.save`, `file.saveAs`, `file.saveCopy` | with the `backgroundSave` / `backgroundExport` preferences on (the default) the desktop app writes the file from a snapshot on a worker thread: the call returns `{path, background: true, status}` at once, `ui.inspect` lists the jobs still running in `background` (labels such as `"Saving a.vectorcraft"`), and the document counts as saved (as of the snapshot) once its file is written. Quit and Close wait for them. With the preferences off (`prefs.set`), saves return once written |
 
 | `ui.dialog.*` on `importPdf` | `{field: "page" \| "allPages" \| "range" \| "cropTo" \| "password", value}` | opening (`app.open`, `file.open`) a PDF or `.ai` with several pages or a password, or placing one (`file.place` without `page`), opens the Import PDF dialog (`mode`: `open` or `place`; `pageCount`). While `__locked` is true, set `password` and `ui.dialog.confirm` to unlock it (a one-page file then opens or is placed). Then set `allPages: false` and `range` ("2-3, 5") to open some pages, or `page` to place one, and `cropTo` (`bounding`, `art`, `crop`, `trim`, `bleed`, `media`); `ui.dialog.confirm` opens or places them (placing keeps the other `file.place` params, such as `link`, `template` and `at`, in `__place`). `document.open {pages, cropTo, password}` and `file.place {page, crop, password}` do the same without the dialog |