a `classonlymethod` decorator

```py
class A:
@classmethod
def a(cls): ...
@classonlymethod
def b(cls): ...
A.a() # ok
A.b() # ok
A().a() # ok
A().b() # not ok (😞)
```
The reasoning is that the `classmethod` decorator is often used for alternative constructors, so using it on instances would be a mistaken usage.

### implementation
We could attempt a vanilla implementation using descriptors, otherwise a simple alias
```py
if not TYPE_CHECKING:
classonlymethod = classmethod
else:
class classonlymethod(classmethod): # or something
...
```

No file, test, or entry point is named in the issue. Start by locating the runtime decorator definitions and existing typing tests, then use the A example to verify class access remains valid while instance access is rejected. Done means the runtime and type-checking behavior for classonlymethod is agreed and covered.

Written by the indexing model from the issue text.