Infer exact types

```py
class A: ...
A() or 1 # error, we know that it's not a subtype that has an `__eq__`
reveal_type(A()) # Exactly[A]
def f(a: A):
a or 1 # no error
reveal_type(a) # A
```
```py
class A: ...
class B(A): ...
isinstance(A(), B) # error
reveal_type(A()) # Exactly[A]
def f(a: A):
isinstance(a, B) # no error
reveal_type(a) # A
```

Would provide a lot of benefits in terms of 'type tightness'(strict-equality).

Start from the two examples in the issue and trace how basedmypy currently infers concrete class instances and checks `or` and `isinstance` expressions. Establish the intended `Exactly[A]` behavior for direct expressions and parameters, then add coverage for both cases before resolving how it should interact with subtype checks.

Written by the indexing model from the issue text.