Add-ons: Artwork Cleanup — simplify keeping shape, health check, deep ungroup, rejoin

A built-in add-on under Object › Plug-ins › Artwork Cleanup:
- Simplify Anchors (Keep Shape) removes anchors while the outline stays
  within a tolerance, optionally keeping sharp corners.
- Document Health Check finds and fixes 16 kinds of problems in the
  selection or the document (stray points, duplicate and collinear
  anchors, tiny handles, nearly closed paths, needless groups, compound
  paths and clipping groups, unpainted and invisible objects, duplicate
  objects, art off every artboard, empty artboards…), with Show, Fix
  Selected and Fix All, each fix one undo step.
- Deep Ungroup and Rejoin Path Pieces.

The add-on lives in its own modules (pathops, engine and ui-egui
addons folders) and leaves the existing commands, tools and menus as
they were.

crates/engine/src/cmd.rs

@@ -1,5 +1,6 @@
 //! The command registry. Ids follow Illustrator's menu structure.
 
+mod addons;
 pub(crate) mod appearance;
 mod assets;
 mod attributes;
@@ -322,6 +323,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(perspgrid::specs());
         v.extend(shortcutset::specs());
         v.extend(importmatches::specs());
+        v.extend(addons::specs());
         v
     })
 }