Rebuild: 8 more ranges being fetched than the connections need

Ranges with a chunk or two left take fewer than perRange connections:
with loops/perRange + 2 ranges, 3.2 of 40 loops were idle on average
over 15 minutes (up to 9.5).

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

app/services/prefetcher.ts

@@ -354,8 +354,9 @@ export class Prefetcher {
     const room = this.spoolBytes < coordinatorConfig.spool.maxBytes
     // With at most perRange connections a range, enough ranges still being fetched for all loops: fully
     // spooled ones count as ready but need no connection (30 of 40 busy on small ranges; LOG.md 2026-10-10 (2)).
+    // 8 more than that: ranges with a chunk or two left take fewer than perRange (with +2, 3 of 40 loops idle).
     const { perRange } = coordinatorConfig.spool
-    const short = perRange > 0 ? Math.ceil(this.loops / perRange) + 2 - (leased.length + started.length) : 0
+    const short = perRange > 0 ? Math.ceil(this.loops / perRange) + 8 - (leased.length + started.length) : 0
     const want = Math.max(ahead - Number(ready), short)
     const fresh = room && want > 0
       ? await rows(db.query()).where('r.status', 'planned').where('r.spooled_chunks', 0).whereRaw('r.next_id <= r.end_id').orderBy([{ column: 'r.priority' }, { column: 'r.id' }]).limit(want)