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/.gitignore

@@ -0,0 +1,48 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.build/
+.buildlog/
+.history
+.svn/
+.swiftpm/
+migrate_working_dir/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins-dependencies
+.pub-cache/
+.pub/
+/build/
+/coverage/
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release
+
+# Widget Preview related
+.widget_preview/