feat(engine): add HelpPlugin (H/?) and Challenge cleared toast

- HelpPlugin: full-window cheat sheet listing every keybinding,
  toggled with H or ?. Three unit tests cover open/close/slash.
- AnimationPlugin: ChallengeAdvancedEvent now surfaces as a
  3-second "Challenge N cleared!" toast.

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

solitaire_engine/src/lib.rs

@@ -7,6 +7,7 @@ pub mod challenge_plugin;
 pub mod daily_challenge_plugin;
 pub mod events;
 pub mod game_plugin;
+pub mod help_plugin;
 pub mod input_plugin;
 pub mod layout;
 pub mod progress_plugin;
@@ -32,6 +33,7 @@ pub use events::{
     NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
 };
 pub use game_plugin::{GameMutation, GamePlugin};
+pub use help_plugin::{HelpPlugin, HelpScreen};
 pub use input_plugin::InputPlugin;
 pub use layout::{compute_layout, Layout, LayoutResource};
 pub use resources::{DragState, GameStateResource, SyncStatus, SyncStatusResource};