Stack overflow on divergently recursive type statement with typevar with itself as the default

### Summary

Found this while just messing around. This code:
```py
type A[T = A] = A[int]
```
Gives a stack overflow like this `thread '<unknown>' (27632) has overflowed its stack` when run through ty (and crashes the playground).

### Version

ty 0.0.18 (751672710 2026-02-20) playground dec65ad3c