M8.6: Puppet Warp pins appear at once, warp from the rest shape and follow Undo

- Pins show as soon as the tool has art selected: automatic ones at the
  centre and the middle of each limb's end (at least three), until pins are
  placed. They stay across tool switches while the selection is unchanged and
  a new selection starts afresh.
- The pins live in the document (Document.puppet, never saved) with the rest
  shape they started from: every warp starts from it, so warps don't stack and
  dragging a pin back restores the original; Undo/Redo take the pins back with
  the art. Adding and deleting pins are undo steps too.
- object.puppetWarp gains rest: true (the tool's pins); the new query
  object.puppetWarp.pins reads them. A click off the mesh adds nothing and
  says so (not-allowed cursor, canvas label); the command refuses such pins.
- The mesh grid covers the expanded art, so tiny art with a wide Expand no
  longer allocates millions of cells.
- Tests on groups, compound paths and open strokes.

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

crates/engine/src/lib.rs

@@ -1049,6 +1049,10 @@ impl Session {
         f(&st.doc, &mut st.selection);
         st.selection.prune(&st.doc);
         st.revision += 1;
+        // Puppet Warp pins belong to the art they were placed on: another selection starts afresh.
+        if st.doc.puppet.as_ref().is_some_and(|p| p.ids != st.selection.objects) {
+            cmd::distortcmds::drop_puppet_pins(st);
+        }
         Ok(())
     }