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/ios/Runner/Base.lproj/LaunchScreen.storyboard

@@ -0,0 +1,37 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="12121" systemVersion="16G29" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" launchScreen="YES" colorMatched="YES" initialViewController="01J-lp-oVM">
+    <dependencies>
+        <deployment identifier="iOS"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="12089"/>
+    </dependencies>
+    <scenes>
+        <!--View Controller-->
+        <scene sceneID="EHf-IW-A2E">
+            <objects>
+                <viewController id="01J-lp-oVM" sceneMemberID="viewController">
+                    <layoutGuides>
+                        <viewControllerLayoutGuide type="top" id="Ydg-fD-yQy"/>
+                        <viewControllerLayoutGuide type="bottom" id="xbc-2k-c8Z"/>
+                    </layoutGuides>
+                    <view key="view" contentMode="scaleToFill" id="Ze5-6b-2t3">
+                        <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
+                        <subviews>
+                            <imageView opaque="NO" clipsSubviews="YES" multipleTouchEnabled="YES" contentMode="center" image="LaunchImage" translatesAutoresizingMaskIntoConstraints="NO" id="YRO-k0-Ey4">
+                            </imageView>
+                        </subviews>
+                        <color key="backgroundColor" red="1" green="1" blue="1" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
+                        <constraints>
+                            <constraint firstItem="YRO-k0-Ey4" firstAttribute="centerX" secondItem="Ze5-6b-2t3" secondAttribute="centerX" id="1a2-6s-vTC"/>
+                            <constraint firstItem="YRO-k0-Ey4" firstAttribute="centerY" secondItem="Ze5-6b-2t3" secondAttribute="centerY" id="4X2-HB-R7a"/>
+                        </constraints>
+                    </view>
+                </viewController>
+                <placeholder placeholderIdentifier="IBFirstResponder" id="iYj-Kq-Ea1" userLabel="First Responder" sceneMemberID="firstResponder"/>
+            </objects>
+            <point key="canvasLocation" x="53" y="375"/>
+        </scene>
+    </scenes>
+    <resources>
+        <image name="LaunchImage" width="168" height="185"/>
+    </resources>
+</document>