接入移动壳文本文件导出能力

Expo 移动壳通过文件系统写入缓存文本并调用系统分享保存面板

补充移动壳导出能力依赖、配置守卫和 HostBridge 单测

更新宿主壳能力协议、方案文档和共享决策记录

apps/mobile-shell/package.json

@@ -15,8 +15,10 @@
     "@expo/metro-runtime": "^56.0.15",
     "expo": "^56.0.12",
     "expo-clipboard": "^56.0.4",
+    "expo-file-system": "^56.0.8",
     "expo-haptics": "^56.0.3",
     "expo-linking": "^56.0.14",
+    "expo-sharing": "^56.0.18",
     "expo-status-bar": "^56.0.4",
     "react": "^19.0.0",
     "react-native": "^0.86.0",