refactor(app): extract build_app(), add CoreGamePlugin placeholder (closes #42, closes #44)

- Split run() into build_app(sync_provider) -> App and run()
- Add empty CoreGamePlugin registered in build_app()
- Issue #43 closed via API (main.rs already satisfies it)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_app/src/lib.rs

@@ -29,17 +29,27 @@ use solitaire_data::{load_settings_from, provider_for_backend, settings_file_pat
 use solitaire_engine::{
     register_theme_asset_sources, AchievementPlugin, AnalyticsPlugin, AnimationPlugin, AssetSourcesPlugin,
     AudioPlugin, AutoCompletePlugin, AvatarPlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
-    CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin,
-    FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
-    OnboardingPlugin, PausePlugin, PlayBySeedPlugin, ProfilePlugin, ProgressPlugin,
-    RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin, SafeAreaInsetsPlugin,
-    SelectionPlugin, SettingsPlugin,
-    SplashPlugin, StatsPlugin, SyncPlugin, SyncSetupPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin,
-    TimeAttackPlugin, UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin,
-    WinSummaryPlugin,
+    CoreGamePlugin, CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin,
+    FeedbackAnimPlugin, FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin,
+    LeaderboardPlugin, OnboardingPlugin, PausePlugin, PlayBySeedPlugin, ProfilePlugin,
+    ProgressPlugin, RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin,
+    SafeAreaInsetsPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin, StatsPlugin, SyncPlugin,
+    SyncProvider, SyncSetupPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin, TimeAttackPlugin,
+    UiFocusPlugin, UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
-/// App entry point — builds and runs the Bevy app.
+fn load_settings() -> Settings {
+    settings_file_path()
+        .map(|p| load_settings_from(&p))
+        .unwrap_or_default()
+}
+
+/// Build the Bevy app without entering the event loop.
+pub fn build_app(sync_provider: Box<dyn SyncProvider + Send + Sync>) -> App {
+    build_app_with_settings(load_settings(), sync_provider)
+}
+
+/// App entry point — configures runtime services, builds, and runs the app.
 ///
 /// Called from both the desktop `bin` target's `main` shim and (on
 /// Android) the platform's NativeActivity / GameActivity glue.
@@ -68,12 +78,15 @@ pub fn run() {
         );
     }
 
-    // Load settings before building the app so we can construct the right
-    // sync provider. Falls back to defaults if no settings file exists yet.
-    let settings: Settings = settings_file_path()
-        .map(|p| load_settings_from(&p))
-        .unwrap_or_default();
+    let settings = load_settings();
     let sync_provider = provider_for_backend(&settings.sync_backend);
+    build_app_with_settings(settings, sync_provider).run();
+}
+
+fn build_app_with_settings(
+    settings: Settings,
+    sync_provider: Box<dyn SyncProvider + Send + Sync>,
+) -> App {
 
     // Restore the previous window geometry if the player has one saved.
     // Otherwise open at the platform default (1280×800, centred on the
@@ -214,7 +227,8 @@ pub fn run() {
         .add_plugins(UiFocusPlugin)
         .add_plugins(UiTooltipPlugin)
         .add_plugins(SplashPlugin)
-        .add_plugins(DiagnosticsHudPlugin);
+        .add_plugins(DiagnosticsHudPlugin)
+        .add_plugins(CoreGamePlugin);
 
     // On Android the default WinitSettings use UpdateMode::Continuous for
     // the focused window, which means Bevy renders as fast as possible even
@@ -257,7 +271,7 @@ pub fn run() {
         app.add_systems(Update, apply_smart_default_window_size);
     }
 
-    app.run();
+    app
 }
 
 /// One-shot Update system that runs only on launches without saved