- In `shaper::refresh`, replace full document walks with `changed_shaper_groups`,
diffing `before` and `after` layers and short-circuiting on pointer-equal (`Arc::ptr_eq`)
subtrees in O(1).
- Return whether Shaper groups rebuilt or modified artwork (`Result<bool>`),
allowing `Session::edit` to skip the second `cmd::objects_on_path::refresh`
call when no artwork was modified.
- In `textwrap::refresh`, check pointer equality on layers against `before.layers`,
skipping untouched layers in O(1) and checking only modified layers.