The default grouping surface: 1px hairline border, near-black fill, 12px radius.

```jsx
<Card interactive sheen>
  <CardContent>
    <Icon name="Shield" size={24} />
    <CardTitle style={{marginTop:12}}>Private by default</CardTitle>
    <CardDescription>Every request is scoped to your org.</CardDescription>
  </CardContent>
</Card>
```

Fills: default (grid tiles) · emphasis (featured) · quiet (landing story cards) · plain. Hover only ever brightens the border — cards never lift, scale, or glow.