design how to interact with `SpecialForm`s

```py
from typing import get_args

def literal_to_tuple(f: TypeForm[Literal[Unpack[Ts]]]) -> tuple[Unpack[Ts]]:
return get_args(f)

ValidKey = Literal["A", "B", "C"]
valid_keys = literal_to_tuple(ValidKey) # type is ("A", "B", "C")
```

Something like this I think

Start with the example using typing.get_args, Literal, Unpack, TypeForm, and the proposed literal_to_tuple helper. Determine and document how SpecialForms should be interacted with, including the intended behavior and scope; the issue does not identify files, tests, or a completed design.

Written by the indexing model from the issue text.