🐛`fixed` properties not respected by `toTyped()`

The `toTyped` API doesn't respect `fixed` properties on classes, resulting in the following unexpected outcome:

```pkl
class Bird {
fixed type = "Parrot"
name = "Polly"
}

untypedBird {
type = "Cocker spaniel"
name = "Morris"
}

result = untypedBird.toTyped(Bird)
```

[produces](https://pkl-playground.vercel.app/?share=truck-species-screen)

```
untypedBird {
type = "Cocker spaniel"
name = "Morris"
}
result {
type = "Cocker spaniel"
name = "Morris"
}
```

Start by reproducing the provided Pkl example in the linked playground and trace the toTyped API behavior for fixed properties. The work is done when converting untypedBird to Bird preserves Bird's fixed type value instead of accepting the input value.

Written by the indexing model from the issue text.