feat(engine): line-bomb booster clears a row or column

lib/game/engine/game_engine.dart

@@ -236,4 +236,18 @@ class GameEngine {
     _checkStuck();
     return true;
   }
+
+  /// Booster: empties one row or one column (exactly one of [row]/[col]).
+  /// No move/score/objective effect. Re-checks stuck.
+  bool useLineBomb({int? row, int? col}) {
+    if (_phase == GamePhase.won || _phase == GamePhase.lost) return false;
+    if ((row == null) == (col == null)) return false; // need exactly one
+    for (var i = 0; i < GridState.size; i++) {
+      final x = col ?? i;
+      final y = row ?? i;
+      _grid = _grid.withCell(x, y, const Cell(CellType.empty));
+    }
+    _checkStuck();
+    return true;
+  }
 }

test/game/engine/booster_test.dart

@@ -66,4 +66,39 @@ void main() {
     e.declineAndLose();
     expect(e.useShuffle(), isFalse);
   });
+
+  test('useLineBomb(row:) empties that row, no scoring or move', () {
+    final e = GameEngine(_stage()); // row 0 has cells at x=0,1
+    final score0 = e.score;
+    final moves0 = e.movesUsed;
+
+    final ok = e.useLineBomb(row: 0);
+
+    expect(ok, isTrue);
+    for (var x = 0; x < 8; x++) {
+      expect(e.grid.isOccupied(x, 0), isFalse, reason: 'col $x');
+    }
+    expect(e.score, score0);
+    expect(e.movesUsed, moves0);
+  });
+
+  test('useLineBomb(col:) empties that column', () {
+    final e = GameEngine(_stage()); // (0,0) filled
+    expect(e.useLineBomb(col: 0), isTrue);
+    for (var y = 0; y < 8; y++) {
+      expect(e.grid.isOccupied(0, y), isFalse, reason: 'row $y');
+    }
+  });
+
+  test('useLineBomb requires exactly one of row/col', () {
+    final e = GameEngine(_stage());
+    expect(e.useLineBomb(), isFalse);
+    expect(e.useLineBomb(row: 0, col: 0), isFalse);
+  });
+
+  test('useLineBomb is rejected after the attempt finishes', () {
+    final e = GameEngine(_stage());
+    e.declineAndLose();
+    expect(e.useLineBomb(row: 0), isFalse);
+  });
 }