narrowing gets very broken with intersections

```py
class A: pass
class B: pass
class C: pass

x: A
assert isinstance(x, B | C)
assert isinstance(x, C)
reveal_type(x) # A & C
```

Start by reproducing the narrowing behavior with the Python snippet in the issue and inspect basedmypy’s type-narrowing and intersection handling. Determine the intended revealed type after the two isinstance assertions, then add a regression test showing the corrected result.

Written by the indexing model from the issue text.