feat(engine): auto-complete — cards auto-deal to foundations

When is_auto_completable flips true (stock/waste empty, all cards
face-up), AutoCompletePlugin fires MoveRequestEvent every 120 ms,
driving cards to the foundation one at a time without player input.
An "Auto-completing…" toast announces the sequence.

- solitaire_core: add next_auto_complete_move() to GameState with
  3 new unit tests
- solitaire_engine: new AutoCompletePlugin with detect + drive systems
  and 4 unit tests; animation_plugin shows one-shot toast on activation
- solitaire_app: register AutoCompletePlugin

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

solitaire_engine/src/lib.rs

@@ -2,6 +2,7 @@
 
 pub mod achievement_plugin;
 pub mod animation_plugin;
+pub mod auto_complete_plugin;
 pub mod audio_plugin;
 pub mod card_plugin;
 pub mod challenge_plugin;
@@ -32,6 +33,7 @@ pub use daily_challenge_plugin::{
 pub use progress_plugin::{LevelUpEvent, ProgressPlugin, ProgressResource, ProgressUpdate};
 pub use weekly_goals_plugin::{WeeklyGoalCompletedEvent, WeeklyGoalsPlugin};
 pub use animation_plugin::{AnimationPlugin, CardAnim};
+pub use auto_complete_plugin::AutoCompletePlugin;
 pub use audio_plugin::{AudioPlugin, AudioState, SoundLibrary};
 pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{