docs(canvas): explain compute workers and display resource lifetimes

src/lib/canvas-runtime/digest.worker.ts

@@ -1,7 +1,14 @@
+/**
+ * 单请求摘要 Worker 入口。只接收 version/id/Blob，输出摘要或稳定错误码，不拥有任何模型、凭证或持久任务。
+ * 宿主在完成/取消时 terminate；accepted 防止一个专用 Worker 误被重复消息启动第二份计算。
+ */
 import {digestWorkerSource} from './digest-worker-source.ts';
 // Dedicated to one request. Host terminates on success/failure/abort; no storage, model or credentials here.
+/** 将 globalThis 限定为本 Worker 使用的消息接口，避免引入浏览器页面组件依赖。 */
 const scope=globalThis as unknown as {onmessage:((event:MessageEvent)=>void)|null;postMessage(value:unknown):void};
+/** 首条有效请求被接受后保持 true，直到整个 Worker 被宿主终止。 */
 let accepted=false;
+/** 校验协议和请求后启动只读摘要；成功/失败都回显输入 ID 与字节数，供宿主核对。 */
 scope.onmessage=event=>{
   const request=event.data;if(accepted||!request||request.version!==1||!Number.isSafeInteger(request.id)||request.id<1||!(request.bytes instanceof Blob))return;accepted=true;
   void digestWorkerSource(request.bytes).then(sha256=>scope.postMessage({version:1,id:request.id,byteLength:request.bytes.size,sha256}),()=>scope.postMessage({version:1,id:request.id,byteLength:request.bytes.size,error:'digest_worker_failed'}));