接入原生壳文本文件导入能力

新增 file.importText HostBridge 契约和 H5 facade

移动端通过 Expo DocumentPicker 读取受控文本文件

桌面端通过 Tauri 文件选择框读取受控文本文件

更新壳能力检查、测试、方案文档和共享决策记录

apps/mobile-shell/package.json

@@ -15,6 +15,7 @@
     "@expo/metro-runtime": "^56.0.15",
     "expo": "^56.0.12",
     "expo-clipboard": "^56.0.4",
+    "expo-document-picker": "^56.0.4",
     "expo-file-system": "^56.0.8",
     "expo-haptics": "^56.0.3",
     "expo-image-picker": "^56.0.18",