收紧文本导出载荷边界

将文本导出载荷预校验提升到共享 HostBridge 契约
让 H5 facade 按共享文本导出边界归一化文件名、MIME 和内容大小
保留 Expo 与 Tauri 壳对文本字节数和 MIME 的二次校验
增加文本导出载荷测试和三端壳总门禁反查
同步宿主壳协议文档和共享决策记录

scripts/check-native-shells.mjs

@@ -1060,6 +1060,18 @@ function assertH5HostBridgePayloadBoundaries() {
       'H5 HostBridge facade must normalize share.open payloads with the shared share boundary',
     );
   }
+  if (
+    !h5HostBridgeSource.includes(
+      'const normalizedPayload = normalizeHostBridgeExportTextPayload(params);',
+    ) ||
+    !h5HostBridgeSource.includes(
+      "'file.exportText',\n      normalizedPayload,",
+    )
+  ) {
+    throw new Error(
+      'H5 HostBridge facade must normalize file.exportText payloads with the shared text export boundary',
+    );
+  }
   if (
     !h5HostBridgeSource.includes(
       'const normalizedPayload = normalizeHostBridgeExportImagePayload(params);',