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/lib.rs

@@ -1,5 +1,6 @@
 //! Bevy integration layer for Solitaire Quest.
 
+pub mod achievement_plugin;
 pub mod animation_plugin;
 pub mod card_plugin;
 pub mod events;
@@ -10,6 +11,7 @@ pub mod resources;
 pub mod stats_plugin;
 pub mod table_plugin;
 
+pub use achievement_plugin::{AchievementPlugin, AchievementsResource};
 pub use animation_plugin::{AnimationPlugin, CardAnim};
 pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{
@@ -20,5 +22,5 @@ pub use game_plugin::{GameMutation, GamePlugin};
 pub use input_plugin::InputPlugin;
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};
-pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen};
+pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
 pub use table_plugin::{PileMarker, TableBackground, TablePlugin};