Fixes #1463

We'll have to bail in some cases that we didn't before to fix this. This
seems fine though. The core problem was that given this theme:
```css
@theme {
  --radius: calc(var(--radius));
}
```

We'd see this CSS:
```css
.rounded {
  border-radius: calc(var(--radius));
}
```

And we'd try to replace variables in `calc(var(--radius))`. This doesn't
_start_ with a var(…) so our recursion guards didn't catch this.

This entire system is a bit fragile and I'll be working to clean this up
along with how we handle pixel equivalents, light-dark replacements,
color-mix replacements, etc… in some future PRs (#1330 is one of those
but I want to reorganize + tweak some code first in an earlier PR which
will make that one "nicer")