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_data/src/lib.rs

@@ -61,8 +61,8 @@ pub use stats::{StatsExt, StatsSnapshot};
 
 pub mod storage;
 pub use storage::{
-    cleanup_orphaned_tmp_files, load_stats, load_stats_from, save_stats, save_stats_to,
-    stats_file_path,
+    cleanup_orphaned_tmp_files, delete_game_state_at, game_state_file_path, load_game_state_from,
+    load_stats, load_stats_from, save_game_state_to, save_stats, save_stats_to, stats_file_path,
 };
 
 pub mod achievements;