u_strCompareIter compares two UCharIterator texts in code unit or code
point order as ICU 78.1's ustring.cpp does, driving the guest iterators
through their move, next, current and previous callbacks in ICU's order.
u_getUnicodeVersion reports Unicode 17.0, the data version of ICU 78.1.
A null callback that ICU would call throws std::invalid_argument, and the
final difference wraps instead of overflowing. The debt entry records
that neither export was compared with a console.
