Snapshot<T> class

A container class that holds both the previous and current values during a state change.

The Snapshot class is used throughout Fluxivity to represent state transitions. It's passed to stream listeners and effect callbacks to provide context about what changed.

Example usage:

final counter = Reactive(0);

// Add an effect that can access both old and new values
counter.addEffect((Snapshot<int> snapshot) {
  print('Counter changed from ${snapshot.oldValue} to ${snapshot.newValue}');
});

counter.value = 1; // Prints: "Counter changed from 0 to 1"