hapticLevelForGroupSize function
Maps a just-popped group's size to a haptic level (pure, testable) — use this once real group/pop matching rules exist.
Implementation
HapticLevel hapticLevelForGroupSize(int size) {
if (size >= 8) return HapticLevel.heavy;
if (size >= 4) return HapticLevel.medium;
return HapticLevel.light;
}