Add model change detection to the widget API

Instead of taking `&mut state`, notify the model of updates using `DerefMut`/`Deref` traits.

Notify the model of updates using `DerefMut`/`Dere` traits.

Let's say we want to implement a demo like [this WebGPU demo](https://kai.graphics/webgpu-samples/?sample=animometer): we're animating n bodies and there is a slider to select how many. When we move the slider, we recreate the scene (expensive). Otherwise, we just animate the scene (cheap).

If we were to do this using Bevy/Egui integration, you'd have the `count` stored in the ECS and a system would detect changes to it and recreate the scene. Except egui wants `&mut count`, which through the `DerefMut` trait always marks the value as changed and you would always recreate the scene. The standard workaround is this:

```rust

#[derive(Resource)]
struct SliderValue(f32);

fn ui_system(
mut contexts: EguiContexts,
mut val: ResMut,
) {
Window::new("Demo Slider").show(contexts.ctx_mut(), |ui| {
let new_val = *val.0;
ui.add(Slider::new(&mut new_val, 0.0..=1.0));
if new_val != *val {
*val = new_val;
}
});
}
```

If the slider widget took `DerefMut` then rendering the slider would not trigger modification updates.

Start by locating the widget API and the existing model-change detection behavior, then inspect how mutable widget values are passed through Rust's Deref and DerefMut traits. Done means a slider can update its value without falsely notifying the model on every render, while genuine model changes are still detected.

Written by the indexing model from the issue text.