feat(engine): add AchievementPlugin with persistent unlock tracking

On GameWonEvent, build an AchievementContext from StatsResource + GameState
+ wall-clock hour, evaluate ALL_ACHIEVEMENTS, flip newly-satisfied records
to unlocked, persist atomically, and emit AchievementUnlockedEvent for
each new unlock. AnimationPlugin's toast resolves the event's ID to the
achievement's display name via achievement_plugin::display_name_for.

Introduces StatsUpdate system set so AchievementPlugin can reliably run
after StatsResource reflects the win. AchievementPlugin::headless() used
in tests to avoid touching ~/.local/share/solitaire_quest/achievements.json.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/animation_plugin.rs

@@ -5,6 +5,7 @@
 
 use bevy::prelude::*;
 
+use crate::achievement_plugin::display_name_for;
 use crate::card_plugin::CardEntity;
 use crate::events::{AchievementUnlockedEvent, GameWonEvent};
 use crate::game_plugin::GameMutation;
@@ -126,7 +127,7 @@ fn handle_achievement_toast(
     for ev in events.read() {
         spawn_toast(
             &mut commands,
-            format!("Achievement: {}", ev.0),
+            format!("Achievement: {}", display_name_for(&ev.0)),
             ACHIEVEMENT_TOAST_SECS,
         );
     }