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>

start/env.ts

@@ -69,6 +69,8 @@ export default await Env.create(new URL('../', import.meta.url), {
   FETCH_CONCURRENCY: Env.schema.number.optional(),
   BODY_CONCURRENCY: Env.schema.number.optional(),
   COLLECTION_CONCURRENCY: Env.schema.number.optional(),
+  ARCHIVE_WALK_CONCURRENCY: Env.schema.number.optional(),
+  ARCHIVE_WALK_MAX_SEGMENTS: Env.schema.number.optional(),
   DEFERRED_SETTLE_MS: Env.schema.number.optional(),
   DEFERRED_EXPAND_MAX_PARTS: Env.schema.number.optional(),
   DEFERRED_CONCURRENCY: Env.schema.number.optional(),