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/lib.rs

@@ -31,11 +31,11 @@
 pub mod app;
 pub mod core;
 pub mod handler;
+pub mod math;
 pub mod pipeline;
 pub mod resources;
 pub mod scene;
 pub mod utils;
-pub mod math;
 
 /// Re-export of the high-level application facade for convenient top-level access.
 /// Users create App instances via `AppBuilder`, then call `.run(handler)` to start the application.