fix(lib): migrate to winit 0.30 ApplicationHandler model

winit 0.30.13 removed WindowBuilder and deprecated EventLoop::run. Move
window/GPU creation into ApplicationHandler::resumed, expose AppHandler::setup
hook, switch App::run to run_app, and migrate both examples. pollster becomes a
regular dependency (used by app.rs).

lib/src/handler.rs

@@ -24,6 +24,12 @@ use crate::core::Frame;
 /// render (draw call execution). Default implementations provide empty update and automatic
 /// scene rendering for convenience.
 pub trait AppHandler {
+    /// Called once by `App::run`, right after the window/GPU context are created (winit `resumed`).
+    /// Use it to register shaders, build Meshes/Materials, and populate `app.scene` before the loop
+    /// starts. This replaces the pre-`run` setup that was possible before the winit 0.30 migration.
+    /// Default implementation does nothing.
+    /// Inputs: app — mutable reference to the fully-initialized App facade.
+    fn setup(&mut self, _app: &mut App) {}
     /// Called once per frame before rendering begins. Used for physics updates, input processing,
     /// entity management, and any other pre-render logic. Default implementation does nothing.
     /// Inputs: _app — mutable reference to the App facade providing access to all subsystems.