Visibility api

```py
@public
def f(): ...

@internal
def g(): ...
```
and so on and so forth.

The issue only provides a Python example using @public and @internal, with no implementation files, tests, or acceptance criteria named. First clarify the intended visibility API and then locate the relevant basedmypy typing or checker entry points; completion should include defined behavior and tests.

Written by the indexing model from the issue text.