An `Exact` annotation modifier

```py
class A: ...
class B(A): ...
def f(a1: Exact[A], a2: A):
a1 or 1 # error
a2 or 1 # no error

f(A(), A())
f(B(), B()) # error: expected A
```
```py
class A(Generic[T]):
def __init__(t: Exact[T]): ...

A[int](1) # ok
A[int](True) # error
```
```py
def test(a: T, b: Exact[T]): ...

o: object
test(o, "a") # error, expected exactly 'object' found 'str'
```

The issue names no repository files, tests, or entry points; begin by tracing how annotations and generic argument compatibility are checked. Use the three examples as acceptance cases, including rejecting subclasses and incompatible inferred types.

Written by the indexing model from the issue text.