Umbrella crate that re-exports the A2UI backends under the historical
a2ui::core / a2ui::tui paths, so existing use a2ui::core::... and
use a2ui::tui::... imports keep working after the workspace split.
The Slint backend is available as a2ui::slint under the slint cargo
feature — it is opt-in because it pulls the (heavy) Slint runtime.