feat(core): expose frame view and auto-render the scene

- AppHandler::render now receives the current &Frame; its default
  implementation renders the whole scene automatically via
  app.render_scene(frame.view()) (Option A). Users can simply not
  implement render for full auto-rendering.
- Add Renderer::render_scene: batch-renders every scene entity in a
  single render pass. Factored per-mesh draw logic into a private
  draw_entity helper shared with Renderer::render.
- Add App::render_scene(view) delegating to the Renderer.
- Fill simple.rs with a real quad (mesh/material/entity) without
  importing wgpu; the scene now auto-renders via the trait default.

lib/src/handler.rs

@@ -17,18 +17,23 @@
 //! the engine "brick by brick" through direct Context/PipelineCache/Renderer manipulation if needed.
 
 use crate::app::App;
+use crate::core::Frame;
 
 /// Trait defining user-provided game logic injected into the render loop at two callback points.
 /// Users implement this trait to define what happens per-frame: update (pre-render logic) and
-/// render (draw call execution). Default implementations provide empty update for convenience.
+/// render (draw call execution). Default implementations provide empty update and automatic
+/// scene rendering for convenience.
 pub trait AppHandler {
     /// 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.
     fn update(&mut self, _app: &mut App) {}
-    /// Called during each RedrawRequested event after frame acquisition. Used for executing draw calls
-    /// by iterating Scene entities and calling app.renderer.render(view, mesh, material) per entity.
-    /// Must be implemented — called every frame that needs rendering.
-    /// Inputs: app — mutable reference to the App facade providing access to all subsystems.
-    fn render(&mut self, app: &mut App);
+    /// Called during each RedrawRequested event after frame acquisition, receiving the current frame.
+    /// Used for custom draw call execution. Default implementation renders the whole scene
+    /// automatically (`app.render_scene(frame.view())`), so most users don't need to override it.
+    /// Advanced users override this method to control drawing manually.
+    /// Inputs: app — mutable reference to the App facade; frame — the acquired frame exposing its view.
+    fn render(&mut self, app: &mut App, frame: &Frame) {
+        app.render_scene(frame.view());
+    }
 }