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/headers/file_key.ts

@@ -1,4 +1,4 @@
-import { createHash } from 'node:crypto'
+import { hash } from 'node:crypto'
 
 /**
  * Identity of one uploaded file: every segment of it has the same poster, the
@@ -7,7 +7,7 @@ import { createHash } from 'node:crypto'
  * See CONCEPT.md (D12).
  */
 export function fileKey(poster: string, base: string, totalSegments: number): string {
-  return createHash('sha1').update(`${poster}\n${base}\n${totalSegments}`).digest('hex')
+  return hash('sha1', `${poster}\n${base}\n${totalSegments}`)
 }
 
 /** True when a yEnc name looks like a real filename (it has an extension), not a random token. */
@@ -26,7 +26,7 @@ export function looksLikeFilename(name: string): boolean {
  */
 export function weakFileKey(name: string, size: number, totalParts: number): string {
   const identity = looksLikeFilename(name) ? `name\n${name}` : 'shape'
-  return createHash('sha1').update(`weak\n${identity}\n${size}\n${totalParts}`).digest('hex')
+  return hash('sha1', `weak\n${identity}\n${size}\n${totalParts}`)
 }
 
 /**
@@ -41,7 +41,5 @@ export function counterFileKey(segment: { poster: string; subject: string; total
   const counter = /\[(\d+)\/(\d+)\]/.exec(segment.subject)
   if (!counter || segment.totalParts < 2) return null
   const day = segment.postedAt && !Number.isNaN(segment.postedAt.getTime()) ? segment.postedAt.toISOString().slice(0, 10) : ''
-  return createHash('sha1')
-    .update(`counter\n${segment.poster}\n${Number(counter[1])}/${Number(counter[2])}\n${segment.totalParts}\n${day}`)
-    .digest('hex')
+  return hash('sha1', `counter\n${segment.poster}\n${Number(counter[1])}/${Number(counter[2])}\n${segment.totalParts}\n${day}`)
 }