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

src/services/host-bridge/hostBridge.ts

@@ -1,4 +1,6 @@
 import type {
+  FileExportImagePayload,
+  FileExportImageResult,
   FileExportTextPayload,
   FileExportTextResult,
   HapticsImpactPayload,
@@ -73,6 +75,8 @@ export type HostShareGridRequest = {
 
 export type HostFileExportTextRequest = FileExportTextPayload;
 
+export type HostFileExportImageRequest = FileExportImagePayload;
+
 export type HostClipboardWriteTextRequest = {
   text: string;
 };
@@ -687,6 +691,27 @@ export async function exportHostTextFile(
   }
 }
 
+export async function exportHostImageFile(
+  params: HostFileExportImageRequest,
+) {
+  if (!canUseNativeHostCapability('file.exportImage')) {
+    return false;
+  }
+
+  try {
+    return await requestNativeAppHostBridge<FileExportImageResult>(
+      'file.exportImage',
+      params,
+      { timeoutMs: 30000 },
+    );
+  } catch (error) {
+    if (isUnsupportedHostBridgeError(error)) {
+      return false;
+    }
+    throw error;
+  }
+}
+
 export async function requestHostHapticsImpact(
   params: HostHapticsImpactRequest = {},
 ) {