Switch to Flutter (Android + iOS) for iPhone support; port engine to Dart

아이폰 지원 요구로 Android 전용 Kotlin 스택을 크로스플랫폼 Flutter로 교체.
- app/: Flutter 프로젝트(android+ios). 코어 로직은 순수 Dart 모듈 app/lib/engine.dart
  로 이식(SRS+180 킥·7-bag·홀드·고스트·락딜레이·T-spin/mini·콤보/B2B). 웹/Kotlin 판과 동일 알고리즘.
- app/test/engine_test.dart: Dart 테스트 24개 전부 통과, flutter analyze 0 issue.
- 이전 android/(Kotlin) 모듈 제거(Flutter로 대체). README에 Flutter 전환 + iOS 빌드 제약(맥 필요) 명시.
- 게임 UI(목업 ② + §11 터치)와 설치용 빌드는 다음 단계.

app/README.md

@@ -0,0 +1,17 @@
+# tetrig
+
+A new Flutter project.
+
+## Getting Started
+
+This project is a starting point for a Flutter application.
+
+A few resources to get you started if this is your first Flutter project:
+
+- [Learn Flutter](https://docs.flutter.dev/get-started/learn-flutter)
+- [Write your first Flutter app](https://docs.flutter.dev/get-started/codelab)
+- [Flutter learning resources](https://docs.flutter.dev/reference/learning-resources)
+
+For help getting started with Flutter development, view the
+[online documentation](https://docs.flutter.dev/), which offers tutorials,
+samples, guidance on mobile development, and a full API reference.