show "narrowed from" for members

```py
class A:
a: object
class B(A):
a: int
a: A
a = B()
reveal_type(a.a) # N: Revealed type is "int"
# Actually expected N: Revealed type is "int" (narrowed from "object")
```

Start by reproducing the shown Python example with classes A and B and the reveal_type(a.a) check. Trace the type-checker path that produces the revealed member type, then update its output so it reports "int" narrowed from "object" and verify the example's expected note.

Written by the indexing model from the issue text.