Slint backend for A2UI.
Translates an A2UI component tree (the flat id → ComponentModel map owned
by [a2ui_base::model]) into a Slint reactive tree and bridges Slint UI
events back to the framework-agnostic interaction layer in a2ui_base.
Unlike the ratatui backend (immediate-mode painting on a character grid),
Slint is retained-mode + declarative: layout is handled by Slint's engine,
so this backend does not reproduce the tui's measure pass or
layout_engine. Instead it walks the component tree into a [live_tree]
model that .slint components bind to reactively.
Everything here lives behind the backend cargo feature, which pulls in the
Slint runtime. Without it this crate is an empty shell (it compiles with no
dependencies beyond a2ui-base), keeping the workspace's default build light.