M4.35: native files compress, save atomically, drop unused images, keep newer keys and save down to v1

- Format v3: optional gzip (Preferences > File Handling > Use Compression, or
  document.save {compress}); compressed files open by their magic bytes, legacy
  ones too.
- Every file write goes to a temporary file renamed over the target (engine
  writes and the desktop app's writer), so a failed save keeps the old file.
- Only the images the document uses are written.
- Document keys from newer versions are kept (Document.extra, flattened).
- Assigned colour profiles are a real field (Document.color_profiles),
  migrated from unknown.colorProfiles on load.
- document.save {version: 1|2} writes files older versions open; {preview}
  embeds a PNG of the first artboard of at most 256 px.

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

docs/mcp.md

@@ -159,6 +159,13 @@ and reflected or repeated gradients (`spreadMethod`) are expanded into stops. `<
 found in the SVG's folder) stay linked images (see Linked images); SVG files and `data:` SVGs become art; a file that
 can't be read becomes a placeholder in its box, named in `warnings` and `missingLinks`.
 
+Native files: `run_command document.save {compress: true}` writes a gzip-compressed `.vectorcraft` (the
+`useCompression` preference makes that the default for saves; compressed files open like any other, told apart by
+their content). `version: 2` or `1` writes a file older VectorCraft versions open (format name `drawcraft`, never
+compressed), and `preview: true` embeds a PNG of the first artboard, at most 256 px on its longer side. A save writes
+a temporary file and renames it over the old one, so a failed save never damages the file; only the images the
+document uses are written, and document keys written by a newer version are kept.
+
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