Expand: selected symbol instances become their art, as Break Link places it

Object > Expand with Object on now turns each symbol instance in the
selection, including one in a selected group or in a placed document's
art, into its art, as Break Link to Symbol places it. It goes one level
deep: the instances in that art stay instances. The art keeps the
instance's id and place in the stacking order, and Expand stays one
undo step.

- Instances with effects stay instances, and Expand Appearance bakes
  them as before. Instances with an effect on a fill or stroke,
  instances in perspective, and those in blends, repeats and Intertwine
  groups stay instances.
- The art keeps the instance's opacity mask, as Break Link gives it.
- A hidden or locked instance's art stays hidden or locked.
- A placed document in an instance's art is expanded too.
- When the instances' art would make more than 1,000,000 objects, they
  stay instances, the limit Break Link uses.
- The Fill and Stroke options (object.expand.info, and the Expand
  dialog) count the fills, strokes and gradients in the art of those
  instances, with a dynamic instance's own changes applied. The info
  reports under symbolArt which of them only that art holds, and the
  dialog enables those options only while Object is on.

crates/engine/src/tests/placed_document.rs

@@ -851,6 +851,24 @@ fn expand_turns_a_placed_document_into_editable_art_with_its_resources() {
     assert_eq!(info["object"], false, "nothing left to expand: {info}");
 }
 
+/// Expand also expands a placed document in the art of a selected symbol instance, in the same
+/// command.
+#[test]
+fn expand_expands_a_placed_document_in_a_symbol_instances_art() {
+    let dir = Folder::new("placed-expand-symbol");
+    let src = dir.file("badge.vectorcraft");
+    star_source(&src, "star");
+    let mut s = session();
+    let id = place(&mut s, &src);
+    s.select(|_, sel| sel.set([id])).unwrap();
+    let inst = NodeId(s.execute("symbol.new", &json!({"name": "Badge"})).unwrap()["id"].as_u64().unwrap());
+    s.execute("object.expand", &json!({})).unwrap();
+    let doc = &s.doc().unwrap().doc;
+    let mut placed = 0;
+    doc.node(inst).unwrap().walk(&mut |c| placed += usize::from(matches!(c.kind, NodeKind::PlacedDocument(_))));
+    assert_eq!(placed, 0, "the placed document in the art is expanded");
+}
+
 #[test]
 fn saving_a_document_refreshes_the_open_documents_that_place_it() {
     let dir = Folder::new("placed-refresh");