Add scalable font size import

prototype / demo https://codepen.io/argyleink/pen/Yzgbgzm

```css
@layer demo {
.scaleable {
h1, h2, h3, h4 {
font-size: calc(1rem * pow(var(--scale, 1.4), var(--heading-scale)));
}

h1 { --heading-scale: 4 }
h2 { --heading-scale: 3 }
h3 { --heading-scale: 2 }
h4 { --heading-scale: 1 }
}
}
```

Start with the linked CodePen prototype and compare its scalable heading-size rules with the repository's existing font-size imports. Identify the appropriate import entry point, then verify that the scalable font sizes are available and match the prototype behavior.

Written by the indexing model from the issue text.