xor method
Combines this spec with another using XOR logic (exclusive OR).
Equivalent to using the ^ operator. Returns a new spec that is satisfied
when exactly one of the two specs is satisfied, but not both and not neither.
Use this for mutually exclusive conditions.
Example:
// Exactly one must be true
final exclusive = IsStudent().xor(IsWorking());
final status = IsPublished().xor(IsDraft()); // One or the other, not both
Implementation
Spec<T> xor(Spec<T> other) => _Xor(this, other);