接入原生壳分享卡图片导出

新增 file.exportImage 宿主能力契约

分享卡下载在原生壳中优先走宿主图片导出

Expo 壳写入缓存图片并调用系统分享保存

Tauri 壳通过保存对话框写入图片字节

补齐能力漂移检查、测试和架构文档

apps/mobile-shell/scripts/check-config.mjs

@@ -118,8 +118,10 @@ for (const dependency of ['expo-file-system', 'expo-sharing']) {
 
 for (const snippet of [
   'file.exportText',
+  'file.exportImage',
   'Sharing.shareAsync',
   'normalizeHostBridgeExportFileName',
+  'base64Data',
 ]) {
   if (!bridgeSource.includes(snippet)) {
     throw new Error(`mobile shell HostBridge missing ${snippet}`);
@@ -140,6 +142,7 @@ for (const capability of [
   'app.openExternalUrl',
   'clipboard.writeText',
   'file.exportText',
+  'file.exportImage',
   'haptics.impact',
 ]) {
   if (!mobileCapabilitySet.has(capability)) {