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_engine/src/core_game_plugin.rs

@@ -0,0 +1,15 @@
+//! Central plugin that groups all gameplay systems.
+//!
+//! Register [`CoreGamePlugin`] once in the app instead of the individual
+//! plugins. Systems are added here rather than directly in the app entry point.
+
+use bevy::prelude::*;
+
+/// Groups all Ferrous Solitaire gameplay plugins.
+pub struct CoreGamePlugin;
+
+impl Plugin for CoreGamePlugin {
+    fn build(&self, _app: &mut App) {
+        // Gameplay systems will be migrated here in follow-up issues.
+    }
+}

solitaire_engine/src/lib.rs

@@ -19,6 +19,7 @@ pub mod daily_challenge_plugin;
 pub mod difficulty_plugin;
 pub mod diagnostics_hud;
 pub mod events;
+pub mod core_game_plugin;
 pub mod game_plugin;
 pub mod help_plugin;
 pub mod home_plugin;
@@ -66,6 +67,7 @@ pub use analytics_plugin::{AnalyticsPlugin, AnalyticsResource};
 pub use challenge_plugin::{
     challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,
 };
+pub use core_game_plugin::CoreGamePlugin;
 pub use daily_challenge_plugin::{
     DailyChallengeCompletedEvent, DailyChallengePlugin, DailyChallengeResource,
 };
@@ -154,6 +156,7 @@ pub use stats_plugin::{
     ReplayPrevButton, ReplaySelectorCaption, SelectedReplayIndex, StatsPlugin, StatsResource,
     StatsScreen, StatsUpdate, WatchReplayButton,
 };
+pub use solitaire_data::SyncProvider;
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};
 pub use sync_setup_plugin::SyncSetupPlugin;
 pub use ui_focus::{Disabled, FocusGroup, Focusable, FocusedButton, UiFocusPlugin};