When mixing vars with MQs, vars fallback if not redeclared

Nobody has claimed this yet.

Quite a confusing title, I apologise. Essentially, if a variable is declared and altered in a MQ and then mixed in the same declaration as one that isn't, it falls back to its original value.

```
:root {
--gutter: 1rem;
--gutterLarge: 2rem;

@include mq(tablet) {
--gutter: 1.5rem;
--gutterLarge: 3rem;
}

@include mq(desktop) {
--gutterLarge: 4rem;
}
}

div {
padding: var(--gutter) var(--gutterLarge);
}
```

This will output:

```
div {
padding: 1rem 2rem;
}

@media (min-width: 46.25em) {
div {
padding: 1.5rem 3rem;
}
}

@media (min-width: 61.25em) {
div {
padding: 1rem 4rem;
}
}
```

I would expect the last one to be `1.5rem 4rem`.

Start by reproducing the provided Sass-like input and comparing the generated CSS at the desktop breakpoint with the reported expectation. Trace the variable resolution and media-query handling from the plugin's entry point; done means the desktop output retains `--gutter`'s tablet value while applying the desktop `--gutterLarge` value, without regressing the other output.

Written by the indexing model from the issue text.