Smiley face part 1
This example renders a static smiley face in an SVG using React and D3. The face is centered in a 960×500 viewport, with a yellow background circle, black stroke, two eye circles, and a mouth drawn as an arc path. The code defines a `BackgroundCircle` React component and uses the `arc()` generator from d3 to create the mouth’s path geometry. All shapes are positioned via a group transform, while constants like `eyeOffsetX`, `eyeRadius`, and `mouthRadius` control the facial feature placement.