Complete archive listings by walking RAR headers, at low priority

Inspection lists only the entries whose headers are in the first segment
of a first volume. A walk hops from header to header, fetching one segment
per entry and volume, in its own queue (one job per collection worker) on
NNTP connections inspection isn't waiting for. content:walk-archives queues
releases inspected earlier (CONCEPT D40).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/rar/index.ts

@@ -3,3 +3,4 @@
  * contents from the first segment of a first volume. See CONCEPT.md (D19).
  */
 export { parseRarHeaders, rarFormat, decodeRar4Name, storedEntryData, type RarInfo, type RarEntry } from './parse.js'
+export { walkRar, type VolumeReader, type RarWalk } from './walk.js'