Feature Request: Support for CSS Color Level 5 (`color-mix()` and Relative Color Syntax)

Nobody has claimed this yet.

Modern browsers have already implemented [CSS Color Module Level 5](https://www.w3.org/TR/css-color-5/).

Would it be possible to support the newly added `color-mix()` and Relative Color Syntax?

## `color-mix()` example

```css
/* Basic: Mixing two colors in srgb space with percentages */
.mix-basic {
background-color: color-mix(in srgb, red 30%, blue 70%);
}

/* Omission: Second percentage is omitted (automatically calculated as 85%) */
.mix-omitted {
background-color: color-mix(in oklab, #007bff 15%, black);
}

/* Hue interpolation method for cylindrical color spaces */
.mix-hue {
background-color: color-mix(in hsl longer hue, hsl(10deg 100% 50%), hsl(150deg 100% 50%));
}
```

## Relative Color Syntax

```css
/* Basic: Destructuring and overriding alpha (opacity) */
.rcs-alpha {
background-color: rgb(from var(--primary) r g b / 0.5);
}

/* Math: Modifying channels using calc() (e.g., creating a lighter variant) */
.rcs-calc {
background-color: hsl(from #007bff h s calc(l + 20%));
}

/* Channel keyword swapping (e.g., using 'r' channel value for 'g' and 'b') */
.rcs-swap {
color: rgb(from oklch(0.6 0.25 120) r r r);
}
```

No files, tests, or entry points are named. Start by locating the CSS color parsing and serialization entry points, then compare the issue's color-mix() and Relative Color Syntax examples with current support. Done means both syntax families, including omitted percentages, hue interpolation, channel references, and calc() expressions, are supported and covered by tests.

Written by the indexing model from the issue text.