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/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),
         };