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_app/src/main.rs

@@ -1,6 +1,7 @@
 use bevy::prelude::*;
 use solitaire_engine::{
-    AnimationPlugin, CardPlugin, GamePlugin, InputPlugin, StatsPlugin, TablePlugin,
+    AchievementPlugin, AnimationPlugin, CardPlugin, GamePlugin, InputPlugin, StatsPlugin,
+    TablePlugin,
 };
 
 fn main() {
@@ -21,5 +22,6 @@ fn main() {
         .add_plugins(InputPlugin)
         .add_plugins(AnimationPlugin)
         .add_plugins(StatsPlugin::default())
+        .add_plugins(AchievementPlugin::default())
         .run();
 }