Part of #868: Save As .ai offers Legacy (version 7), with Transparent Images and an artboard range

aiVersion 7 saves through the legacy writer, one file per artboard; range picks the artboards
and document.save writes them again. Appearances are expanded first. Transparent images
(see-through pixels, opacity or a blending mode) are flattened with the art under them into
opaque images clipped to where they change it (transparentImages "flatten", the default), or
left out ("omit"); both warn.

crates/engine/src/cmd/effectcmd.rs

@@ -407,7 +407,7 @@ fn expand_pieces(d: &mut Document, m: &mut Node, all: bool, stroke_art: effects:
 /// Expand Appearance of `id` (and, recursively, of a group's or layer's members): every fill and
 /// stroke becomes an object of its own (strokes outlined, brushes as their art), geometry effects
 /// are baked, raster effects become an embedded image, and the object keeps its transparency.
-fn expand_node(d: &mut Document, id: NodeId, brushes: &[vectorcraft_brush::Brush], out: &mut Vec<NodeId>) {
+pub(crate) fn expand_node(d: &mut Document, id: NodeId, brushes: &[vectorcraft_brush::Brush], out: &mut Vec<NodeId>) {
     let Some(mut n) = d.node(id).cloned() else { return };
     if !expandable(&n) {
         return;