Reviewer noted that `< 4` only catches underflow; programs leaving 5+
values on the DYADIC stack silently passed isValid(). Change the guard
to `!== 4` so both under- and overflow are rejected, matching the
documented 'exactly 4 outputs' contract.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Replace silent ?? '0' fallbacks with an explicit length check that
throws when the DYADIC stack holds fewer than 4 values at program
termination. isValid() in the evolution pipeline now correctly
rejects underflow programs instead of silently scoring them as valid
with zeroed outputs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
