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/game_plugin.rs

@@ -37,6 +37,7 @@ impl Plugin for GamePlugin {
             .add_event::<StateChangedEvent>()
             .add_event::<GameWonEvent>()
             .add_event::<crate::events::CardFlippedEvent>()
+            .add_event::<crate::events::AchievementUnlockedEvent>()
             .add_systems(
                 Update,
                 (