Rebuild: a lease never waits for prefetching; one-shot hashes

A lease was only granted for a range with spooled chunks, and the
prefetcher kept ranges ahead per active lease: once the spool ran dry a
worker got none and slept 30 s, fewer ranges were leased, fewer
prefetched. Three workers held one lease between them (~50k headers/s).
A lease now takes a spooled range first, else the next planned one
(leased ranges are fetched first; the stream waits).

crypto.hash instead of createHash().update().digest() for file and
release keys: same digests, ~2.5x faster (~4.6% of a worker's time).

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

lib/edge/range_indexer.ts

@@ -1,4 +1,4 @@
-import { createHash } from 'node:crypto'
+import { hash } from 'node:crypto'
 import type { Overview } from '../nntp/index.js'
 import { FileAggregator, type AggregatedFile, isArchiveStart, isLaterVolume, sevenZipVolume } from '../files/index.js'
 import { RuleSet, isRandomFile, uploadTool, type Rules } from '../rules/index.js'
@@ -281,7 +281,7 @@ export class RangeIndexer {
 
 /** The release key; a file without a recognisable release becomes a release of its own. */
 export function releaseKeyOf(s: EdgeSegment): string {
-  return s.file.release?.key ?? createHash('sha1').update(`file\n${s.fileKey}`).digest('hex')
+  return s.file.release?.key ?? hash('sha1', `file\n${s.fileKey}`)
 }
 
 function postedOf(segments: EdgeSegment[]): number | null {