feat(engine): add AnimationPlugin with slide, cascade, and toast (Phase 3F)

- CardAnim component lerps cards from old to new position on every move
- card_plugin now adds CardAnim instead of teleporting cards on state change
- Snap-back on invalid drag reuses the same mechanism (StateChangedEvent)
- Win cascade flies all 52 cards off-screen with staggered delay on GameWonEvent
- Achievement toast scaffold wired to AchievementUnlockedEvent (Phase 5 content)
- Fix input_plugin test: click Queen's visible strip, not geometric centre

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/events.rs

@@ -41,3 +41,9 @@ pub struct GameWonEvent {
 /// Fired when a card's face-up state changes during gameplay.
 #[derive(Event, Debug, Clone, Copy)]
 pub struct CardFlippedEvent(pub u32);
+
+/// Achievement unlocked notification — name of the achievement.
+///
+/// Uses `String` as a placeholder; replaced with `AchievementRecord` in Phase 5.
+#[derive(Event, Debug, Clone)]
+pub struct AchievementUnlockedEvent(pub String);