Fix exchange-only process continuation with explicit append path

crates/oa-desktop/src/behavior/topology/direct.rs

@@ -30,6 +30,7 @@ pub enum CanvasEvent {
     Scrolled(Context, scenario::ScrollArea, iced::Vector),
     Choose(Context, CanvasTool),
     Select(Context, Id),
+    AppendExchange(Context),
     Begin {
         context: Context,
         tool: Tool,
@@ -67,6 +68,7 @@ impl CanvasEvent {
             | Self::Expand(c, ..)
             | Self::Open(c, ..)
             | Self::Back(c)
+            | Self::AppendExchange(c)
             | Self::Zoom(c, ..)
             | Self::Viewport(c, ..) => *c,
             Self::Begin { context, .. }
@@ -204,6 +206,15 @@ impl State {
         }
         Ok(())
     }
+    pub(crate) fn append_exchange(&mut self, p: &Project, owner: Id) -> Result<(), String> {
+        let last = legacy_order(p, owner)
+            .last()
+            .copied()
+            .ok_or("Choose an ordered exchange path to append to.")?;
+        self.update(p, Event::Start(owner, Tool::Exchange, Some(last)))?;
+        self.draft.as_mut().expect("started form").fixed = true;
+        Ok(())
+    }
     pub(crate) fn connect(
         &mut self,
         p: &Project,