feat(Notifications): swap snackbars after minDisplayMs dwell

NotificationList now replaces the active snackbar with the next queued one
once it has been visible for `minDisplayMs` (default 1000ms), instead of
waiting for the full `duration` to elapse.

- Same-type repeated triggers refresh the active snackbar and reset its
  auto-dismiss timer (bypass the dwell).
- Persistent variants (no `duration`) are never displaced by transient
  feedback; only a newer persistent or an external dismissal can replace
  them.
- Selection policy is pluggable via the new `pickNext` prop
  (`PickNextNotification`); the default `defaultPickNext` is FIFO so
  nothing is silently dropped. `pickOldest` and `pickNewest` are exported
  as building blocks for custom strategies.

Author: MartinCupela