feat(engine): hammer booster removes one cell, no scoring

lib/game/engine/game_engine.dart

@@ -216,4 +216,15 @@ class GameEngine {
     }
     _phase = GamePhase.lost;
   }
+
+  /// Booster: empties one filled cell. No move/score/combo/objective effect.
+  /// Allowed only mid-attempt (playing or stuck). Returns false on an empty
+  /// cell or finished attempt so the caller keeps the booster.
+  bool useHammer(int x, int y) {
+    if (_phase == GamePhase.won || _phase == GamePhase.lost) return false;
+    if (!_grid.isOccupied(x, y)) return false;
+    _grid = _grid.withCell(x, y, const Cell(CellType.empty));
+    _checkStuck();
+    return true;
+  }
 }

test/game/engine/booster_test.dart

@@ -0,0 +1,49 @@
+import 'package:block_seasons/game/engine/game_engine.dart';
+import 'package:block_seasons/game/models/stage.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+StageConfig _stage() => StageConfig.fromJson({
+      'id': 'b_test',
+      'seed': 1,
+      'moveLimit': 20,
+      'preset': [
+        {'x': 0, 'y': 0, 't': 'filled', 'c': 3},
+        {'x': 1, 'y': 0, 't': 'filled', 'c': 4},
+      ],
+      'objectives': [
+        {'type': 'reachScore', 'target': 100000},
+      ],
+      'stars': {
+        'two': {'movesLeft': 5},
+        'three': {'movesLeft': 10},
+      },
+    });
+
+void main() {
+  test('useHammer empties a filled cell without scoring or spending a move', () {
+    final e = GameEngine(_stage());
+    final score0 = e.score;
+    final moves0 = e.movesUsed;
+
+    expect(e.grid.isOccupied(0, 0), isTrue);
+    final ok = e.useHammer(0, 0);
+
+    expect(ok, isTrue);
+    expect(e.grid.isOccupied(0, 0), isFalse);
+    expect(e.score, score0, reason: 'no points from a booster');
+    expect(e.movesUsed, moves0, reason: 'no move spent');
+  });
+
+  test('useHammer on an empty cell returns false and changes nothing', () {
+    final e = GameEngine(_stage());
+    expect(e.grid.isOccupied(5, 5), isFalse);
+    expect(e.useHammer(5, 5), isFalse);
+    expect(e.grid.isOccupied(5, 5), isFalse);
+  });
+
+  test('useHammer is rejected once the stage is won or lost', () {
+    final e = GameEngine(_stage());
+    e.declineAndLose();
+    expect(e.useHammer(0, 0), isFalse);
+  });
+}