Rebuild: enough ranges being fetched for every connection

With 2 connections per range the prefetcher needs 20+ ranges with
chunks left; fully spooled ranges counted as ready (SPOOL_AHEAD_PER_LEASE)
but take no connection, so on small ranges 30 of 40 connections were
busy (13-15 ranges). Fresh ranges now also fill up to loops/perRange + 2
ranges being fetched.

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

app/services/prefetcher.ts

@@ -341,8 +341,13 @@ export class Prefetcher {
     const started = await rows(db.query()).where('r.status', 'planned').where('r.spooled_chunks', '>', 0).whereNull('r.spooled_at').orderBy([{ column: 'r.priority' }, { column: 'r.id' }])
     const [{ ready }] = await mine(db.from('ranges as r').where('r.status', 'planned').where('r.spooled_chunks', '>', 0)).count('* as ready')
     const room = this.spoolBytes < coordinatorConfig.spool.maxBytes
-    const fresh = room && Number(ready) < ahead
-      ? 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(ahead - Number(ready))
+    // 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)).
+    const { perRange } = coordinatorConfig.spool
+    const short = perRange > 0 ? Math.ceil(this.loops / perRange) + 2 - (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)
       : []
     const targets: Target[] = []
     for (const row of [...leased, ...(room ? started : []), ...fresh]) {