feat(engine): add resources, events, and GamePlugin event routing

Introduces the plumbing layer for Phase 3: GameStateResource wraps
solitaire_core::GameState, DragState tracks in-progress drags, and
SyncStatusResource holds runtime sync status. GamePlugin routes
Draw/Move/Undo/NewGame request events into GameState and emits
StateChangedEvent and GameWonEvent for downstream systems.

Also adds the Phase 3 implementation plan under docs/superpowers/plans/.

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

solitaire_app/src/main.rs

@@ -1,4 +1,5 @@
 use bevy::prelude::*;
+use solitaire_engine::GamePlugin;
 
 fn main() {
     App::new()
@@ -12,5 +13,6 @@ fn main() {
                 ..default()
             }),
         )
+        .add_plugins(GamePlugin)
         .run();
 }