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/nntp_pool.ts

@@ -75,7 +75,41 @@ class NntpPool {
   }
 
   /** Closes every connection, also those in use: their commands fail. */
+  private plain: Promise<XoverClient> | null = null
+  private plainQueue: Promise<unknown> = Promise.resolve()
+
+  /**
+   * One connection without XFEATURE COMPRESS GZIP, opened when first needed,
+   * one command at a time: for chunks whose compressed reply the server
+   * cuts short (Leases.corrupt). Its replies are deflated here.
+   */
+  async usePlain<T>(work: (client: XoverClient) => Promise<T>): Promise<T> {
+    const run = this.plainQueue.then(async () => {
+      this.plain ??= XoverClient.connect({
+        host: env.get('NNTP_HOST')!,
+        port: env.get('NNTP_PORT', 563),
+        user: env.get('NNTP_USER'),
+        password: env.get('NNTP_PASSWORD'),
+        secure: env.get('NNTP_SECURE', true),
+        compress: false,
+        timeoutMs: coordinatorConfig.nntpTimeoutMs,
+      })
+      const client = await this.plain
+      try {
+        return await work(client)
+      } catch (error) {
+        this.plain = null
+        client.close()
+        throw error
+      }
+    })
+    this.plainQueue = run.catch(() => {})
+    return run
+  }
+
   async close() {
+    void this.plain?.then((c) => c.close()).catch(() => {})
+    this.plain = null
     for (const client of [...this.idle.splice(0), ...this.busy]) client.close()
     this.busy.clear()
     this.open = 0