Add-ons: Smart Pen — draw and edit paths with one tool, panel

A built-in add-on under Object › Plug-ins › Smart Pen:
- The Smart Pen Tool draws and edits Bézier paths without switching
  tools: a click adds the next segment straight or curved (a panel
  toggle; Tab flips the segment being placed or the last one), a drag
  makes a smooth point (Alt breaks the handle, Shift snaps to 45°, Space
  moves the point), a curve after a straight segment joins it tangent.
- On existing paths it selects and moves points, removes a point keeping
  the shape (Alt-click), toggles smooth/corner (double-click), drags
  handles, reshapes a segment by dragging it and inserts a point on it
  (Alt-click); it closes, continues and joins open paths.
- The Smart Pen panel: next segment straight or curved, remove points
  keeping shape, make smooth, make corner, retract handles, close or
  join.

The tool stays out of both toolbars as shipped; the All Tools drawer
lists it under the add-on's heading. Commands are addon.pen.*; the
existing Pen tool, commands and menus are unchanged.

crates/engine/src/cmd/path.rs

@@ -341,7 +341,7 @@ fn set_anchors(s: &mut Session, p: &Value) -> Result<Value> {
 }
 
 /// The compound path enclosing `id`, or `id` itself if it is not inside a compound path.
-fn compound_or_self(d: &Document, id: NodeId) -> NodeId {
+pub(super) fn compound_or_self(d: &Document, id: NodeId) -> NodeId {
     d.parent_of(id).filter(|p| matches!(d.node(*p).map(|n| &n.kind), Some(NodeKind::Compound { .. }))).unwrap_or(id)
 }
 
@@ -547,7 +547,7 @@ fn join_subpaths_in_node(d: &mut Document, id: NodeId, (sa, sb): (usize, usize),
 }
 
 /// Join open paths into a single path, or close an open path, preserving compound path structure.
-fn join(s: &mut Session, p: &Value) -> Result<Value> {
+pub(super) fn join(s: &mut Session, p: &Value) -> Result<Value> {
     const C: &str = "path.join";
     let join_kind = match p.get("to").and_then(Value::as_str) {
         None => None,