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

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

@@ -1,17 +1,25 @@
+/**
+ * 专用 Worker 内的摘要算法选择：小于等于64MiB可用原生 SubtleCrypto，较大文件分块流式 SHA。
+ * 整缓冲读取只允许发生在 Worker，取消由宿主终止该请求的 Worker；本函数不访问存储或文档。
+ */
 import {fault} from '../canvas-core/values.ts';
 import {importSourceDigest} from './import-integrity.ts';
 
 // Each of the browser pool's two active Workers admits at most 64 MiB; larger inputs stream.
+/** 单任务 native 缓冲阈值，不是整个应用内存硬上限；总并发由外部 pool 决定。 */
 export const nativeDigestByteLimit=64*1024*1024;
 
 /** Dedicated Worker only: whole-buffer native hashing never moves onto the UI thread.
  * The host still owns cancellation by terminating the request's Worker. */
+/** 验证 Blob，按能力/大小选 native 或增量实现，均返回相同的完整原字节摘要。 */
 export async function digestWorkerSource(bytes:Blob,subtle:Pick<SubtleCrypto,'digest'>|null|undefined=globalThis.crypto?.subtle):Promise<string>{
  if(!(bytes instanceof Blob))throw fault('invalid_digest_input');
+ // native digest 必须返回32字节；输出固定64字符十六进制，不接受变长响应。
  if(subtle&&bytes.size<=nativeDigestByteLimit){
   const input=await bytes.arrayBuffer(),digest=await subtle.digest('SHA-256',input);
   if(digest.byteLength!==32)throw fault('invalid_digest_response');
   return [...new Uint8Array(digest)].map(value=>value.toString(16).padStart(2,'0')).join('');
  }
+ // Worker fallback 每块1MiB，只做微任务让出；UI 让出策略由 UI fallback 的端口另行决定。
  return importSourceDigest(bytes,undefined,{chunkBytes:1024*1024,yieldTask:()=>Promise.resolve()});
 }