Rebuild: spread prefetch connections over ranges and groups

At most PREFETCH_PER_RANGE (2) connections on a range and
PREFETCH_PER_GROUP (4) on a group; SPOOL_AHEAD_PER_LEASE 2 -> 4 so there
are targets enough. The report says how many ranges and groups are in
flight.

Homelab, 40 connections, alternated 12 min each: unlimited (~10 ranges
in flight) 281k and 167k articles/s; spread (~22 ranges) 319k and 296k,
chunks 11.5 s against 13-24 s.

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

config/coordinator.ts

@@ -58,9 +58,20 @@ export default {
   spool: {
     dir: env.get('SPOOL_DIR', app.tmpPath('spool')),
     /** Ranges kept fetched ahead, per active lease. */
-    aheadPerLease: env.get('SPOOL_AHEAD_PER_LEASE', 2),
+    aheadPerLease: env.get('SPOOL_AHEAD_PER_LEASE', 4),
     /** At least this many ranges ahead, so the first leases start at once. */
     minAhead: env.get('SPOOL_MIN_AHEAD', 2),
     maxBytes: env.get('SPOOL_MAX_GB', 10) * 1e9,
+    /**
+     * At most this many connections on one range, and on one group, at a
+     * time (0: no limit): spreads the requests over ranges and groups instead
+     * of all on the next chunks of the first ranges. Homelab, 40 connections,
+     * 12 min each, alternated: unlimited (on ~10 ranges) 281k and 167k
+     * articles/s, 2 per range and 4 per group (on ~22) 319k and 296k, with
+     * shorter chunk times (11.5 vs 13–24 s; LOG.md 2026-10-09 (18)). Needs
+     * targets for it: aheadPerLease 4.
+     */
+    perRange: env.get('PREFETCH_PER_RANGE', 2),
+    perGroup: env.get('PREFETCH_PER_GROUP', 4),
   },
 }