Toast

Confirms something that just happened and then leaves. Errors the user must act on belong to Alert or AlertDialog.

An instant, consequential action applies immediately plus raises an undo toast — that is the middle reversibility tier.
The two acknowledgement animations are the only springy motion in Reckon, and both are disabled under prefers-reduced-motion.
Toasts stack in one ToastRegion. Never open two regions on one screen.