operator & method
Combines this spec with another using AND logic (&).
Returns a new spec that is satisfied only when both this spec
and other are satisfied by the candidate. Use this when all conditions
must be true. Shorthand for calling and.
Example:
// All conditions must be met
final youngAdult = IsAdult() & IsYoung();
final premium = IsPaid() & HasNoDebts() & IsActive();
Implementation
Spec<T> operator &(Spec<T> other) => _And(this, other);