#1172: share an object's appearance with its undo history until it changes

Every edit of an object copied its whole appearance (its fills, strokes and effects, about
540 bytes for a basic object) into the new node, though a move leaves it as it is. Over 45
whole-selection moves of 10,000 paths, undo history grew by 589 MB; now 349 MB (41 % less), and
a nudge or a drag of the selection takes 12 ms instead of 15.

- Node::appearance is a vectorcraft_doc::Shared<Appearance>: copy-on-write. A clone shares it,
  and the first change through DerefMut copies it (Arc::make_mut). Reads, writes, equality (also
  against a plain Appearance) and the saved form are those of the Appearance itself, so call
  sites read and change it as before. Sites that build or assign a whole appearance add .into(),
  and the few that need a plain copy take (*n.appearance).clone().
- Node::transform changes the appearance only when it has gradients to transform, and pattern
  paints are transformed only when there are some: a move no longer copies an appearance it
  doesn't change.

The workspace suite (5,301 tests) and clippy pass with the change; the doc and engine suites
pass again after the transform guard.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VmMmc1J8XeNLxhRihWv9cc

crates/engine/src/cmd/textedit.rs

@@ -561,7 +561,7 @@ fn create_in_path(s: &mut Session, p: &Value) -> Result<Value> {
         d.remove(pid)?;
         let id = d.alloc_id();
         let mut n = Node::new(id, NodeKind::Text(Box::new(t)));
-        n.appearance = Appearance::default();
+        n.appearance = Appearance::default().into();
         n.opacity = node.opacity;
         n.name = node.name.clone();
         d.insert(par, idx, n)?;