Rebuild: prefetching no longer waits for merging, nor stalls on finished ranges

The coordinator ran the prefetch tick in the same loop as the merger
(up to 30 s a minute): its connections went idle once their targets
were fetched, and the first leases after a start found nothing spooled.
The prefetcher now ticks in a loop of its own.

The prefetcher took the first planned ranges with nothing spooled, even
ones with nothing left to fetch (next_id past the end): two such ranges
(edge:bench's, set back to planned) kept it from fetching anything. It
and the lease query now skip those, and edge:bench marks its range done.

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

commands/edge_bench.ts

@@ -163,6 +163,8 @@ export default class EdgeBench extends BaseCommand {
         peakDetail = `heap ${(m.heapUsed / 1e6).toFixed(0)}/${(m.heapTotal / 1e6).toFixed(0)} MB, external ${(m.external / 1e6).toFixed(0)} MB, array buffers ${(m.arrayBuffers / 1e6).toFixed(0)} MB`
       }
     }
+    // Done, so nothing takes it for a range to work on (a leased one was set back to planned once).
+    if (this.store) await db.from('ranges').where('id', rangeId).update({ status: 'done', done_at: new Date() })
     const cpu = process.cpuUsage(cpuStart)
     const cpuS = (cpu.user + cpu.system) / 1e6
     const total = t.inflate + t.parse + t.index