false errors with `isinstance` and `Intersection`

```py
from basedtyping import Intersection

class A: ...
class B: ...

class C(A, B): ...

AB = Intersection[A, B]

print(isinstance(C(), AB))
```

Start by running the supplied Python reproduction involving basedtyping.Intersection and isinstance, then inspect the type-checking path that reports the false error. Done means the example no longer produces the incorrect diagnostic and a regression test covers this case.

Written by the indexing model from the issue text.