Rebuild: chunks the server's compression cuts short are fetched plain

Eweka sends some ranges' compressed XOVER truncated, the same bytes
every time, while the plain reply is whole (a.b.models 3317215-3367214:
1,675,600 bytes that don't inflate, twice; 50,000 lines plain;
tools/false_end.ts). Not a false end of reply. The worker's corrupt
report removed the chunk and the range was fetched again, with the same
bytes: one range was reported 540 times. (Before this morning's change
the inflate check waited for more data until the 10-minute timeout,
like voyager 1's 'chunk never answered' ranges.)

A reported chunk's start goes into ranges.plain_chunks; the prefetcher
fetches those over one connection without XFEATURE COMPRESS GZIP
(NntpPool.usePlain, opened on demand) and deflates them here. A plain
chunk reported again sets the range aside (status failed, error). The
capture (edge:bench --capture) fetches such a chunk plain too.

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

app/services/prefetcher.ts

@@ -7,6 +7,8 @@ import Spool, { type ChunkSpan } from '#services/spool'
 interface Target {
   id: number
   group: string
+  /** Chunk starts to fetch without compression (ranges.plain_chunks). */
+  plain: Set<number>
   /** Chunks not spooled yet, in order. */
   missing: ChunkSpan[]
   /** All of the range's chunks, from its start. */
@@ -76,10 +78,11 @@ export class Prefetcher {
       const { target, span } = next
       try {
         const fetchedAt = Date.now()
-        const data = await NntpPool.use(async (client) => {
+        const fetch = async (client: Parameters<Parameters<typeof NntpPool.use>[0]>[0]) => {
           await client.group(target.group)
           return client.xoverRaw(span.from, span.to)
-        })
+        }
+        const data = target.plain.has(span.from) ? await NntpPool.usePlain(fetch) : await NntpPool.use(fetch)
         this.stats.chunks++
         this.stats.articles += span.to - span.from + 1
         this.stats.fetchMs += Date.now() - fetchedAt
@@ -134,7 +137,7 @@ export class Prefetcher {
       this.spoolBytes = await Spool.size()
     }
     const mine = (q: any) => (this.shard.count > 1 ? q.whereRaw('r.id % ? = ?', [this.shard.count, this.shard.index]) : q)
-    const rows = (q: any) => mine(q.from('ranges as r').join('groups as g', 'g.id', 'r.group_id').select('r.id', 'g.name as group', 'r.start_id', 'r.end_id', 'r.next_id'))
+    const rows = (q: any) => mine(q.from('ranges as r').join('groups as g', 'g.id', 'r.group_id').select('r.id', 'g.name as group', 'r.start_id', 'r.end_id', 'r.next_id', 'r.plain_chunks'))
     // Leased ranges, checked against the spool's files whatever spooled_at says: a worker waits for them.
     const leased = await rows(db.query()).where('r.status', 'leased').orderBy('r.leased_at')
     // Planned ranges ahead: those started already, then new ones up to SPOOL_AHEAD_PER_LEASE per lease.
@@ -154,7 +157,7 @@ export class Prefetcher {
       for (const span of Spool.chunksOf({ start, end }, Number(row.next_id))) {
         if (!this.claimed.has(`${row.id}:${span.from}`) && !(await Spool.has(Number(row.id), span))) missing.push(span)
       }
-      if (missing.length > 0) targets.push({ id: Number(row.id), group: row.group, missing, total })
+      if (missing.length > 0) targets.push({ id: Number(row.id), group: row.group, plain: new Set((row.plain_chunks ?? []).map(Number)), missing, total })
     }
     return targets
   }