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_engine/src/lib.rs

@@ -1,3 +1,16 @@
-// Bevy plugins are added in Phase 3.
-// This crate will expose: CardPlugin, TablePlugin, AnimationPlugin,
-// AudioPlugin, UIPlugin, AchievementPlugin, SyncPlugin, GamePlugin.
+//! Bevy integration layer for Solitaire Quest.
+//!
+//! Currently exposes `GamePlugin` plus the resources and events it owns.
+//! Additional plugins (`TablePlugin`, `CardPlugin`, `InputPlugin`,
+//! `AnimationPlugin`, etc.) land in later sub-phases of Phase 3.
+
+pub mod events;
+pub mod game_plugin;
+pub mod resources;
+
+pub use events::{
+    CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRequestEvent, NewGameRequestEvent,
+    StateChangedEvent, UndoRequestEvent,
+};
+pub use game_plugin::GamePlugin;
+pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};