feat(engine): shuffle booster re-deals the tray

lib/game/engine/game_engine.dart

@@ -227,4 +227,13 @@ class GameEngine {
     _checkStuck();
     return true;
   }
+
+  /// Booster: re-deals the tray. No move/score effect. Re-checks stuck so a
+  /// dead board with a hopeless tray can become playable again.
+  bool useShuffle() {
+    if (_phase == GamePhase.won || _phase == GamePhase.lost) return false;
+    _tray = _generator.nextTray(_grid);
+    _checkStuck();
+    return true;
+  }
 }

test/game/engine/booster_test.dart

@@ -46,4 +46,24 @@ void main() {
     e.declineAndLose();
     expect(e.useHammer(0, 0), isFalse);
   });
+
+  test('useShuffle replaces the tray without spending a move or scoring', () {
+    final e = GameEngine(_stage());
+    final before = List.of(e.tray);
+    final score0 = e.score;
+    final moves0 = e.movesUsed;
+
+    final ok = e.useShuffle();
+
+    expect(ok, isTrue);
+    expect(e.tray.length, before.length);
+    expect(e.score, score0);
+    expect(e.movesUsed, moves0);
+  });
+
+  test('useShuffle is rejected after the attempt finishes', () {
+    final e = GameEngine(_stage());
+    e.declineAndLose();
+    expect(e.useShuffle(), isFalse);
+  });
 }