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>

tools/false_end.ts

@@ -0,0 +1,33 @@
+/**
+ * Whether a compressed XOVER reply that doesn't inflate is the server's: the
+ * range fetched raw twice (compared with a saved copy), and once uncompressed.
+ *
+ *   node --env-file=.env --import=@poppinss/ts-exec tools/false_end.ts <group> <from> <to> [saved.gz]
+ */
+import { readFileSync } from 'node:fs'
+import { unzipSync } from 'node:zlib'
+import { XoverClient } from '#lib/nntp/index'
+
+const [group, from, to, saved] = process.argv.slice(2)
+const env = process.env
+const connect = (compress: boolean) =>
+  XoverClient.connect({ host: env.NNTP_HOST!, port: Number(env.NNTP_PORT ?? 563), user: env.NNTP_USER, password: env.NNTP_PASSWORD, secure: (env.NNTP_SECURE ?? 'true') !== 'false', compress, timeoutMs: 120_000 })
+const gz = await connect(true)
+await gz.group(group)
+for (let i = 0; i < 2; i++) {
+  const raw = await gz.xoverRaw(Number(from), Number(to))
+  let inflates = 'inflates'
+  try {
+    unzipSync(raw)
+  } catch (e: any) {
+    inflates = `doesn't inflate (${e.message})`
+  }
+  const same = saved ? (Buffer.compare(raw, readFileSync(saved)) === 0 ? ', same bytes as saved' : ', differs from saved') : ''
+  console.log(`compressed #${i + 1}: ${raw.length} bytes, ${inflates}${same}`)
+}
+gz.close()
+const plain = await connect(false)
+await plain.group(group)
+const lines = await plain.xover(Number(from), Number(to))
+console.log(`plain: ${lines.length} overview lines`)
+plain.close()