Bivariant class assignability is broken

### Summary

Ran into this while making my umpteenth generic parser generator soup and messed up a parser generator transformer.
https://play.ty.dev/ba3f04b2-0ba6-46de-a84c-b9e2d792ce89
```py
class Container[T]: ...

def foo(x: Container[str]) -> Container[int]:
    return x  # <- Should error
```

### Version

ty 0.0.35 (bc12d1cad 2026-05-10) playground 7efc9a615