gh-143054: Disallow non-top-level Cut for now (GH-143622)

The behaviour of Cut in nested parentheses, Repeat, Opt, and similar
is somewhat chaotic. Apparently even the academic papers on PEG aren't
as clear as they could be.

And it doesn't really matter. Python only uses top-level cuts.
When that changes, we can clarify as much as necessary (and even
change the implementation to make sense for what we'll need).

Document that this is deliberately unspecified, and add a test to
make sure any decision is deliberate, tested and documented.