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_app/src/main.rs

@@ -1,10 +1,10 @@
 use bevy::prelude::*;
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
-    AchievementPlugin, AnimationPlugin, AudioPlugin, CardPlugin, ChallengePlugin,
-    DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, OnboardingPlugin, PausePlugin,
-    ProgressPlugin, SettingsPlugin, StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin,
-    WeeklyGoalsPlugin,
+    AchievementPlugin, AnimationPlugin, AudioPlugin, AutoCompletePlugin, CardPlugin,
+    ChallengePlugin, DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, OnboardingPlugin,
+    PausePlugin, ProgressPlugin, SettingsPlugin, StatsPlugin, SyncPlugin, TablePlugin,
+    TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -31,6 +31,7 @@ fn main() {
         .add_plugins(CardPlugin)
         .add_plugins(InputPlugin)
         .add_plugins(AnimationPlugin)
+        .add_plugins(AutoCompletePlugin)
         .add_plugins(StatsPlugin::default())
         .add_plugins(ProgressPlugin::default())
         .add_plugins(AchievementPlugin::default())