Let's do this differently.
Here's what I'd like to get to:
import narwhals as nw from narwhals.translate import ( translate_frame, translate_series, to_native, )
dfpd = ... df = nw.DataFrame(df_any)
df = df.with_columns(c = nw.col('a') + nw.col('b'))
result = to_native(df)
we need to just have a single class. can't have all this nonsense...
then, we don't even need a spec...
we can still define entrypoints though?
where should extract native happen?