Idle connections fetch capture ranges into the corpus

The spool only fetches what workers lease soon, in priority order, and the
tail never ends: after a reset the 1,400 capture ranges waited behind it
while 34 of 40 connections idled. Up to CORPUS_AHEAD (8) planned capture
ranges whose chunks aren't all stored are now targets after everything else,
fetched into the corpus only (no spool); a later lease reads them from disk.

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

app/services/corpus.ts

@@ -1,4 +1,4 @@
-import { mkdir, readFile, readdir, rename, rm, writeFile } from 'node:fs/promises'
+import { access, mkdir, readFile, readdir, rename, rm, writeFile } from 'node:fs/promises'
 import { join } from 'node:path'
 import { unzip } from 'node:zlib'
 import { promisify } from 'node:util'
@@ -35,6 +35,24 @@ class Corpus {
     return join(this.rangeDir(range), `${String(index).padStart(4, '0')}-${span.from}-${span.to}.gz`)
   }
 
+  /** Whether every chunk of the range is stored (its range.json). */
+  async complete(range: CorpusRange): Promise<boolean> {
+    if (!this.dir) return false
+    return access(join(this.rangeDir(range), 'range.json')).then(
+      () => true,
+      () => false
+    )
+  }
+
+  /** Whether one chunk is stored. */
+  async has(range: CorpusRange, span: ChunkSpan): Promise<boolean> {
+    if (!this.dir) return false
+    return access(this.path(range, span)).then(
+      () => true,
+      () => false
+    )
+  }
+
   /** A stored chunk, or null. */
   async read(range: CorpusRange, span: ChunkSpan): Promise<Buffer | null> {
     if (!this.dir) return null