feat(engine): in-progress game state persistence

Save game_state.json on app exit and on pause open so players can
resume interrupted sessions. Delete the file on win, loss, or new-game
start. Restore the saved game on launch if it exists and isn't won.

- solitaire_core: add pile_map_serde module so HashMap<PileType,Pile>
  round-trips through JSON (serialized as Vec of pairs)
- solitaire_data: add game_state_file_path, load_game_state_from,
  save_game_state_to, delete_game_state_at with 8 new unit tests
- solitaire_engine/GamePlugin: restore saved game on startup, expose
  GameStatePath resource, save on AppExit, delete on new-game and win
- solitaire_engine/PausePlugin: save on pause open (guards against
  OS-level kills while the overlay is showing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -38,7 +38,7 @@ pub use events::{
     AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, ManualSyncRequestEvent,
     MoveRejectedEvent, MoveRequestEvent, NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
 };
-pub use game_plugin::{GameMutation, GamePlugin};
+pub use game_plugin::{GameMutation, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};