feat(engine): add ProgressPlugin awarding XP on wins with level-up events

On GameWonEvent, computes xp_for_win(time, used_undo) from
solitaire_data, calls PlayerProgress::add_xp, and emits LevelUpEvent
when the level changes. Persists atomically through the configurable
storage path; ProgressPlugin::headless() disables I/O for tests.

Introduces ProgressUpdate system set so future systems can run after
progress mutations.

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

solitaire_app/src/main.rs

@@ -1,7 +1,7 @@
 use bevy::prelude::*;
 use solitaire_engine::{
-    AchievementPlugin, AnimationPlugin, CardPlugin, GamePlugin, InputPlugin, StatsPlugin,
-    TablePlugin,
+    AchievementPlugin, AnimationPlugin, CardPlugin, GamePlugin, InputPlugin, ProgressPlugin,
+    StatsPlugin, TablePlugin,
 };
 
 fn main() {
@@ -23,5 +23,6 @@ fn main() {
         .add_plugins(AnimationPlugin)
         .add_plugins(StatsPlugin::default())
         .add_plugins(AchievementPlugin::default())
+        .add_plugins(ProgressPlugin::default())
         .run();
 }