docs: complete application lifecycle and final validation pass

docs/01-rainbow-triangle.md

@@ -90,7 +90,8 @@ use pollster::block_on;
 use std::sync::Arc;
 use winit::application::ApplicationHandler;
 use winit::dpi::LogicalSize;
-use winit::event::WindowEvent;
+use winit::event::{WindowEvent, ElementState, KeyEvent};
+use winit::event::keyboard::KeyCode;
 use winit::event_loop::{ActiveEventLoop, ControlFlow, EventLoop};
 use winit::window::{Window, WindowId};
 
@@ -136,21 +137,29 @@ impl ApplicationHandler<()> for App {
         }
     }
 
-    fn window_event(
-        &mut self,
-        event_loop_ctl: &ActiveEventLoop,
-        _window_id: WindowId,
-        event: WindowEvent,
-    ) {
-        let Some(state) = self.state.as_mut() else { return };
-        let Some(window) = self.window.as_ref() else { return };
-
+    fn window_event(&mut self, event_loop: &ActiveEventLoop, _window_id: WindowId, event: WindowEvent) {
         match event {
-            WindowEvent::Resized(size) => state.resize(size),
-            WindowEvent::CloseRequested { .. } => event_loop_ctl.exit(),
+            WindowEvent::CloseRequested
+            | WindowEvent::KeyboardInput {
+                event: KeyEvent {
+                    state: ElementState::Pressed,
+                    logical_key: KeyCode::Escape,
+                    ..
+                },
+                ..
+            } => {
+                event_loop.exit();
+            }
+            WindowEvent::Resized(physical_size) => {
+                if let Some(state) = &mut self.state {
+                    state.resize(*physical_size);
+                }
+            }
             WindowEvent::RedrawRequested => {
+                if let Some(state) = &mut self.state {
                     state.render();
-                window.request_redraw();
+                }
+                self.window.as_ref().unwrap().request_redraw();
             }
             _ => {}
         }
@@ -177,6 +186,13 @@ impl ApplicationHandler<()> for App {
 >
 > winit supports `ControlFlow::Poll` (continuous redraw) and `ControlFlow::Wait` (idle until next event). A graphics application needs a steady render loop. `Poll` tells winit to keep firing `RedrawRequested` events. We re-queue ourselves inside the handler via `window.request_redraw()`, matching the wgpu swapchain presentation rhythm.
 
+> **WHY: `ControlFlow` Poll vs Wait — the CPU usage tradeoff**
+>
+> - `ControlFlow::Poll` keeps the render loop running continuously. winit fires `RedrawRequested` as fast as possible, giving you a tight loop ideal for games and animation. The downside: it consumes 100% of one CPU core because the event loop never sleeps.
+> - `ControlFlow::Wait` blocks the event loop until a new event arrives. This is dramatically more CPU-efficient since the loop sleeps between frames. However, it requires explicit `request_redraw()` calls to trigger rendering — without them, the window stays static.
+> - For this tutorial, `ControlFlow::Poll` is used for simplicity: you don't need to reason about when to request redraws. A real application would almost always use `ControlFlow::Wait` with targeted `request_redraw()` calls to avoid wasting CPU cycles.
+> - The `request_redraw()` call in the `RedrawRequested` handler works with both modes, but has different semantics: under `Poll` it keeps the loop going (which is already happening anyway), while under `Wait` it's the sole mechanism that wakes the event loop for the next frame.
+
 **Why `request_redraw()`:** After presenting a frame to the display, we ask
 winit to schedule the next `RedrawRequested` frame. This creates an explicit
 render loop: render → present → request redraw → render → repeat. The rate is