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.
+    }
+}