docs: add window field to State struct for Outdated recovery

docs/01-rainbow-triangle.md

@@ -217,6 +217,7 @@ struct State {
     device: wgpu::Device,
     queue: wgpu::Queue,
     config: wgpu::SurfaceConfiguration,
+    window: Arc<Window>,
     pipeline: wgpu::RenderPipeline,
     vertex_buffer: wgpu::Buffer,
 }
@@ -235,6 +236,12 @@ struct State {
 - **`config`** — holds the surface's current width, height, pixel format, and
   [present mode](concepts/GLOSSARY.md#present-mode). When the window is resized,
   we reconfigure the surface with updated dimensions.
+- **`window`** — shared reference to the winit window. Stored as an `Arc` so
+  the `resize()` method and the `SurfaceError::Outdated` recovery handler can
+  access the window's current dimensions. When the surface becomes outdated
+  (e.g., after a compositor restart or display hotplug), recovery requires
+  reconfiguring the swapchain with the window's live size — and that requires
+  holding a reference to the window itself.
 - **`pipeline`** — the compiled [render pipeline](concepts/GLOSSARY.md#render-pipeline).
   A render pipeline is an immutable configuration combining a shader, a vertex
   buffer layout, a primitive topology, and a [color target](concepts/GLOSSARY.md#color-target)
@@ -389,6 +396,7 @@ impl State {
             device,
             queue,
             config,
+            window: Arc::clone(&window),
             pipeline,
             vertex_buffer,
         })
@@ -964,6 +972,11 @@ fn render(&mut self) {
                     };
                     self.resize(size);
                 }
+                // The window field stored in State enables Outdated recovery:
+                // when the compositor or display server invalidates the surface,
+                // we can reconfigure the swapchain using the window's current
+                // dimensions (self.window.inner_size()) without needing a
+                // separate reference to the window from the event loop.
                 wgpu::SurfaceError::Lost => {
                     log::error!("Surface error: Lost — cannot recover without re-creating State");
                 }