fix(apiproxy): cancel attachment reads during export

Response-consumer cancellation already stopped lineage reads, persistence reads, and ZIP production, but the final attachment phase called readImage without the producer signal. A slow or stalled attachment backend could therefore keep working after the browser abandoned the download and prevent the producer from settling.\n\nExtend the attachment read seam with optional cancellation, forward it through the local backend into Node's filesystem read, and preserve the abort reason rather than wrapping it as a storage failure. The exporter now passes its combined request/consumer signal to every attachment read.\n\nCover both ownership boundaries: the local-store test proves filesystem forwarding and cancellation identity, while the assembled export test cancels a reader during a pending attachment provider call. Regenerate the Cordis API catalog and paired documentation so implementers can rely on the new contract.

packages/attachment/attachment/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/attachment/attachment/README.md
-README.md: 4f450316294e554396adb9a8454051a08d9befd3
-README.zh.md: fe51b0003cdf1659c7c56106b97c6f3139ebe890
+README.md: baeeca0cf939f1a3d4608769b362d532507b90f5
+README.zh.md: 238b90794c510e71fffe34d62b044a5c2ece8a6e

packages/attachment/attachment/README.md

@@ -4,7 +4,7 @@ English | [中文](README.zh.md)
 
 The durable attachment seam. `ctx.attachments` validates and atomically commits immutable image bytes, then returns a serializable `ImageAttachmentRef`; consumers never persist browser paths, object URLs, provider URLs, or base64 in session events.
 
-Unsent composer images remain browser-owned temporary drafts. `validateImage` runs the same admission policy without persisting; batch writers validate every member first so a malformed member cannot strand earlier members as unreferenced objects. `saveImage` commits each accepted image before any model-visible session event is published, and `readImage` verifies the content-addressed object against its logged metadata.
+Unsent composer images remain browser-owned temporary drafts. `validateImage` runs the same admission policy without persisting; batch writers validate every member first so a malformed member cannot strand earlier members as unreferenced objects. `saveImage` commits each accepted image before any model-visible session event is published, and `readImage` verifies the content-addressed object against its logged metadata. Callers may cancel `readImage`; implementations observe cancellation around backend and verification work and preserve it instead of translating it into a storage failure.
 
 ## Model Experience
 

packages/attachment/attachment/README.zh.md

@@ -4,7 +4,7 @@
 
 持久附件服务边界。`ctx.attachments` 校验并以原子方式提交不可变图片字节，随后返回可序列化的 `ImageAttachmentRef`；消费方绝不会在会话事件中持久保存浏览器路径、对象 URL、提供方 URL 或 base64。
 
-未发送的输入区图片仍是由浏览器持有的临时草稿。`validateImage` 运行相同的准入策略，但不执行持久化；批量写入方会先校验每个成员，避免某个格式错误的成员使较早的成员成为无引用对象。`saveImage` 会在发布任何模型可见的会话事件前提交每张已接受的图片，`readImage` 则根据已记录的元数据校验内容寻址对象。
+未发送的输入区图片仍是由浏览器持有的临时草稿。`validateImage` 运行相同的准入策略，但不执行持久化；批量写入方会先校验每个成员，避免某个格式错误的成员使较早的成员成为无引用对象。`saveImage` 会在发布任何模型可见的会话事件前提交每张已接受的图片，`readImage` 则根据已记录的元数据校验内容寻址对象。调用方可以取消 `readImage`；实现会在后端读取与校验工作的边界观察取消，并保留取消语义，而不会将其转换为存储失败。
 
 ## 模型体验
 

packages/attachment/attachment/src/index.ts

@@ -52,9 +52,11 @@ export abstract class AttachmentStore extends Service {
   /**
    * Read one image and verify that bytes still match the recorded reference.
    * @param ref - durable reference from the session log.
+   * @param signal - optional cancellation for backend read and verification work.
    * @returns the verified bytes and canonical reference.
+   * @throws the signal reason when aborted, or a storage error when verification fails.
    */
-  abstract readImage(ref: ImageAttachmentRef): Promise<StoredImageAttachment>
+  abstract readImage(ref: ImageAttachmentRef, signal?: AbortSignal): Promise<StoredImageAttachment>
 }
 
 export default AttachmentStore