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>

tests/unit/services/corpus.spec.ts

@@ -0,0 +1,37 @@
+import { test } from '@japa/runner'
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { gzipSync } from 'node:zlib'
+import coordinatorConfig from '#config/coordinator'
+import Corpus from '#services/corpus'
+
+test.group('Corpus', (group) => {
+  let dir: string
+  let before: string | undefined
+  group.each.setup(async () => {
+    dir = await mkdtemp(join(tmpdir(), 'corpus-'))
+    before = coordinatorConfig.corpus.dir
+    coordinatorConfig.corpus.dir = dir
+  })
+  group.each.teardown(async () => {
+    coordinatorConfig.corpus.dir = before
+    await rm(dir, { recursive: true, force: true })
+  })
+
+  test('knows the chunks it holds, and a range once all are there', async ({ assert }) => {
+    const range = { group: 'alt.binaries.test', start: 1, end: 200_000 }
+    const first = { from: 1, to: 100_000 }
+    const second = { from: 100_001, to: 200_000 }
+    assert.isFalse(await Corpus.has(range, first))
+    await Corpus.write(range, first, gzipSync('1\tsubject\n'), 2)
+    assert.isTrue(await Corpus.has(range, first))
+    assert.isFalse(await Corpus.complete(range))
+    // A reply cut short isn't kept.
+    await Corpus.write(range, second, Buffer.from('not gzip'), 2)
+    assert.isFalse(await Corpus.has(range, second))
+    await Corpus.write(range, second, gzipSync('2\tsubject\n'), 2)
+    assert.isTrue(await Corpus.complete(range))
+    assert.equal((await Corpus.read(range, second))?.length, gzipSync('2\tsubject\n').length)
+  })
+})