Rebuild: hedged chunk fetches and connection max age (both off by default)

A chunk not in after NNTP_HEDGE_AFTER_SECONDS (10) is asked again on one
of NNTP_HEDGE_CONNECTIONS spare connections; the first answer wins and
the other connection is closed (lib/nntp/hedge.ts, tested). Connections
can be retired after NNTP_CONNECTION_MAX_MINUTES (±25%). The prefetch
report counts hedges and slow first requests by connection age.

Homelab, 15 min each: hedging on (40 + 6) 280k articles/s, off 273k.
The second request won about half of its races, but ~25-35% of first
requests were slow (>10 s) at every connection age, so neither helps:
Eweka is slow under our load, not on particular connections. Both
off by default; the age statistics stay.

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

lib/nntp/index.ts

@@ -1,3 +1,4 @@
 export { XoverClient, parseOverviews, type Overview, type XoverClientOptions } from './xover_client.js'
 export { parseXrefGroups } from './xref.js'
 export { parseGroupResponse, type GroupInfo } from './group.js'
+export { hedge, type Request, type Hedged } from './hedge.js'