Added

• Notification API — EzCountdownApi.sendNotification(Notification) fires a one-shot ephemeral timed display from any plugin without creating a persistent countdown or touching countdowns.yml.
• Notification model — lightweight immutable value object with factory methods Notification.ofSeconds(long), Notification.of(Duration), and a fluent NotificationBuilder (display types, format message, start/end messages).
• NotificationBuilder — fluent builder for Notification; all fields have sensible defaults (ACTION_BAR display, {formatted} message); build() validates duration > 0.
• Ephemeral countdown support — Countdown.isEphemeral() / CountdownBuilder.ephemeral(boolean); ephemeral countdowns are removed from memory automatically when they end and are never written to storage.