egui backend for A2UI.
Translates an A2UI component tree (the flat id → ComponentModel map owned
by [a2ui_base::model]) into egui immediate-mode widgets and bridges egui
interactions back to the framework-agnostic interaction layer in a2ui_base.
Unlike the ratatui backend (immediate-mode painting on a character grid with
a manual measure pass) and the Slint backend (retained-mode, forced into a
flat index-array + bounded-depth Node0..N chain because Slint can't
recurse), egui is native-recursive and auto-layouts: the walker in
[walker] recurses directly into a &mut egui::Ui, with no build.rs and
no measure pass. Interactive components (TextField/Slider/CheckBox/…) use
real egui widgets, bridged to the data-model-as-source-of-truth via the
persistent [edit_state::EditBuffers] map.
Everything here lives behind the backend cargo feature, which pulls in the
egui + eframe runtime. Without it this crate is an empty shell (it compiles
with no dependencies beyond a2ui-base), keeping the workspace's default
build light.