Add type selector popup, view root switching, and new type creation

- Type selector chevron [▸] on command row opens searchable popup
- Popup lists all root structs with filter, keyboard nav, side-triangle indicator
- Selecting a type switches the editor view via setViewRootId
- "Create new type" inserts a new root struct with no name
- Command row displays the active view root's name
- Tests for chevron detection, span compatibility, view switching, undo

src/editor.h

@@ -61,6 +61,7 @@ signals:
     void inlineEditCommitted(int nodeIdx, int subLine,
                              EditTarget target, const QString& text);
     void inlineEditCancelled();
+    void typeSelectorRequested();
 
 protected:
     bool eventFilter(QObject* obj, QEvent* event) override;