Rebuild: a deadline per chunk; batches with a huge fragment

A worker waited on one chunk of a leased range for an hour (range
146477: five 15-minute stream timeouts, then the range took 63 s; 88
"stream: Peer disconnected" overnight). The silence timeout doesn't
catch a reply that trickles: a chunk still coming in after
NNTP_CHUNK_DEADLINE_SECONDS (300) is given up, its connection closed,
and fetched again. The report names the oldest request in flight (range,
chunk, age, bytes in) and counts deadlines.

Batches failed with "Maximum call stack size exceeded" (8 overnight):
Math.min(...segments) on a fragment with more segments than the stack
takes arguments.

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

lib/nntp/xover_client.ts

@@ -47,6 +47,10 @@ export class XoverClient {
    */
   lastTiming = { replyMs: 0, bodyMs: 0, longestGapMs: 0 }
   private measuring = false
+  /** Bytes received and not consumed yet: during a reply, how much of it came in. */
+  get buffered() {
+    return this.length
+  }
   private lastDataAt = 0
   private longestGap = 0