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/coordinator_run.ts

@@ -34,9 +34,21 @@ export default class CoordinatorRun extends BaseCommand {
       prefetcher.stop()
       await done
     })
+    this.logger.info('Coordinator started.')
+    // Two loops: the prefetcher's ticks don't wait for merging or planning (a merge round
+    // of up to MERGE_BUDGET_MS left the connections idle once their targets were fetched).
+    const prefetching = (async () => {
+      while (!stopping) {
+        try {
+          await prefetcher.tick()
+        } catch (error: any) {
+          this.logger.error(`Prefetch round failed: ${error.message}`)
+        }
+        await sleep(TICK_MS)
+      }
+    })()
     let plannedAt = 0
     let mergedAt = 0
-    this.logger.info('Coordinator started.')
     while (!stopping) {
       try {
         if (Date.now() - plannedAt >= PLAN_EVERY_MS) {
@@ -65,12 +77,12 @@ export default class CoordinatorRun extends BaseCommand {
           }
           if (totals.completed || totals.incomplete) this.logger.info(`Fragments: ${totals.completed} files completed, ${totals.incomplete} incomplete, ${totals.waiting} waiting.`)
         }
-        await prefetcher.tick()
       } catch (error: any) {
         this.logger.error(`Coordinator round failed: ${error.message}`)
       }
       await sleep(TICK_MS)
     }
+    await prefetching
     while (prefetcher.active > 0) await sleep(200)
     await NntpPool.close()
     stopped!()