Fix #311: Pen: Auto Add/Delete on a selected path

Between paths, the Pen now adds an anchor where a click lands on a
segment of a selected path and deletes the anchor a click lands on,
with the add and delete cursors while hovering, as in the reference
app. The ends of a selected open path still continue it. Shift held,
or General > Disable Auto Add/Delete (a stored preference nothing read
until now, passed to tools as ToolContext::auto_add_delete), starts a
new path instead.

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

crates/engine/src/tests.rs

@@ -252,6 +252,33 @@ fn pen_tool_draws_closed_path() {
     assert!(p.is_closed());
 }
 
+/// Auto Add/Delete: a Pen click on a selected path's segment adds an anchor, unless General →
+/// Disable Auto Add/Delete is on.
+#[test]
+fn pen_click_on_a_selected_path_adds_an_anchor_unless_disabled() {
+    let mut s = session();
+    let v = ViewInfo::default();
+    let r = s.execute("path.create", &json!({"anchors": [{"x": 10, "y": 100}, {"x": 110, "y": 100}, {"x": 210, "y": 100}]})).unwrap();
+    let id = NodeId(r["id"].as_u64().unwrap());
+    s.execute("select.all", &json!({})).unwrap();
+    s.select_tool("pen", v).unwrap();
+    let click = |s: &mut Session| {
+        s.pointer(&PointerEvent::new(PointerKind::Down, 60.0, 101.0), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Up, 60.0, 101.0), v).unwrap();
+    };
+    click(&mut s);
+    let d = &s.doc().unwrap().doc;
+    assert_eq!(d.layers[0].children().unwrap().len(), 1);
+    assert_eq!(d.node(id).unwrap().path_data().unwrap().anchor_count(), 4);
+    s.execute("edit.undo", &json!({})).unwrap();
+    s.execute("prefs.set", &json!({"key": "disableAutoAddDelete", "value": true})).unwrap();
+    s.execute("select.all", &json!({})).unwrap();
+    click(&mut s);
+    let d = &s.doc().unwrap().doc;
+    assert_eq!(d.layers[0].children().unwrap().len(), 2, "a new path starts");
+    assert_eq!(d.node(id).unwrap().path_data().unwrap().anchor_count(), 3);
+}
+
 #[test]
 fn direct_selection_moves_one_anchor() {
     let mut s = session();

crates/engine/src/tooling.rs

@@ -138,6 +138,7 @@ impl Session {
             paste_plain_text: self.prefs.paste_text_formatting == "plain",
             slices_hidden: self.menu.slices_hidden,
             slices_locked: self.menu.slices_locked,
+            auto_add_delete: !self.prefs.disable_auto_add_delete,
             screen: view.screen,
             plane_widget: self.prefs.perspective_widget.show.then_some(self.prefs.perspective_widget.position),
         };