The `—spacing()` function doesn't work in `@property`

**What version of Tailwind CSS are you using?**

v4.1.18

**Reproduction URL**

https://play.tailwindcss.com/pUX02gidgc

**Describe your issue**

```
@property --foo {
  syntax: "<length>";
  inherits: true;
  initial-value: 0; <<< Works as expected. 😀
}
@property --bar {
  syntax: "<length>";
  inherits: true;
  initial-value: --spacing(0); <<< Doesn't work. 😢
}
```