examples: fix simple.rs to compile as the 15-line declarative model

- Replace the broken App::new() (which does not exist) with AppBuilder.
- Strip it down to the PLAN's ~15-line target: no explicit winit/wgpu,
  AppHandler render() left empty (scene rendering still on the roadmap).
- Verified: builds, opens a black window and runs the update->render->present
  loop until closed.
- Update README/PLAN wording now that the simple example compiles (it still
  does not draw a scene).

lib/examples/simple.rs

@@ -1,59 +1,19 @@
-use wsg_lib::resources::{Material, Mesh, Vertex};
-use wsg_lib::utils;
+//! Workflow déclaratif minimal (~15 lignes), sans manipulation WGPU explicite.
+//! `AppBuilder` ouvre la fenêtre, construit le `Context`/`Renderer` et fait tourner la boucle
+//! update → render → present. Le rendu automatisé de la scène n'est pas encore en place
+//! (README, Roadmap étape 1) : `render()` est donc vide pour l'instant.
+use wsg_lib::app::AppBuilder;
+use wsg_lib::utils::WsgError;
 use wsg_lib::{App, AppHandler};
 
-// 1. On définit notre "Jeu" qui implémente le comportement
-struct MonQuad {
-    mesh: Mesh,
-    material: Material,
-}
+struct MonQuad;
 
 impl AppHandler for MonQuad {
-    fn render(&mut self, app: &mut App) {}
+    fn render(&mut self, _app: &mut App) {}
 }
 
 #[pollster::main]
-async fn main() -> Result<(), wsg_lib::utils::WsgError> {
-    // 2. Initialisation via le Builder
-    let mut app = App::new().await?;
-
-    // 3. Setup des ressources (déclaration)
-    app.cache
-        .register_shader("basic", utils::BASIC_SHADER_PATH)
-        .unwrap();
-
-    let vertices: [Vertex; 4] = [
-        Vertex {
-            position: [-0.5, 0.5, 0.0],
-            normal: [0.0, 0.0, 1.0],
-            uv: [0.0, 0.0],
-            color: [1.0, 0.0, 0.0, 1.0],
-        },
-        Vertex {
-            position: [0.5, 0.5, 0.0],
-            normal: [0.0, 0.0, 1.0],
-            uv: [1.0, 0.0],
-            color: [0.0, 1.0, 0.0, 1.0],
-        },
-        Vertex {
-            position: [0.5, -0.5, 0.0],
-            normal: [0.0, 0.0, 1.0],
-            uv: [1.0, 1.0],
-            color: [0.0, 0.0, 1.0, 1.0],
-        },
-        Vertex {
-            position: [-0.5, -0.5, 0.0],
-            normal: [0.0, 0.0, 1.0],
-            uv: [0.0, 1.0],
-            color: [1.0, 1.0, 0.0, 1.0],
-        },
-    ];
-    let indices: [u16; 6] = [0, 1, 2, 0, 2, 3];
-
-    let mesh = Mesh::new(app.context.device(), &vertices, Some(&indices));
-    let material = Material::new(app.renderer.format(), "basic", &mut app.cache);
-
-    // 4. Lancement de la boucle (la magie opère ici)
-    let handler = MonQuad { mesh, material };
-    app.run(handler)
+async fn main() -> Result<(), WsgError> {
+    let app = AppBuilder::new().title("WSG Simple").build().await?;
+    app.run(MonQuad)
 }