merge: fix/b-download

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Cxwgnn2pbzQ7rXv3rq7fei

packages/contracts/src/execution.ts

@@ -223,6 +223,11 @@ export const DownloadTask = EntityBase.extend({
   removedAt: Timestamp.nullable(),
   fileList: z.array(z.object({ path: z.string(), size: NonNegInt, progress: z.number().optional() })).nullable(),
   lastError: JsonObject.nullable(),
+  /** 下载器里连续查不到的起点与轮数:达到阈值才判 removed,中途出现即清零 */
+  missingSince: Timestamp.nullable().optional(),
+  missingCount: NonNegInt.optional(),
+  /** removed 行被再次提交 / 接管复活的次数(完成事件幂等键随之换代) */
+  generation: NonNegInt.optional(),
 });
 export type DownloadTask = z.infer<typeof DownloadTask>;
 export const DownloadTaskCreate = DownloadTask.pick({