(🎁) should `N804` enforce `mcs` on metaclass `classmethod`s?

```py
class M(type):
def f(self): # N804
pass
def m(cls): # no error
pass
@classmethod
def m(cls): # no error
pass
@classmethod
def c(mcs): # N804
pass
```
i would expect this to be `N804`, `no error`, `N804`, `no error`

Start at Ruff's N804 rule implementation and use the provided metaclass snippet as the reproduction case. Verify the four method forms, then confirm that diagnostics match the expected N804 and no-error outcomes.

Written by the indexing model from the issue text.