D3 Smiley Face Part + React
A D3 smiley face built as a React component renders a yellow circle with black stroke, two eye circles, and an arc-shaped mouth using the `arc()` generator from d3-shape. The SVG is sized to the browser window, with the face centered via a `<g>` transform. `ReactDOM.render` mounts the app, while the background circle, eyes, and mouth use fixed dimensions calculated from `width`, `height`, and radius constants. The mouth’s path is produced by `d3.arc()`, with start and end angles defining the open smile.