Content probe: don't download the first segment again for whole files

Par2 sets, NZBs and SFVs were sniffed from their first segment and then
read whole, downloading that segment a second time.

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

tests/unit/probe.spec.ts

@@ -59,7 +59,8 @@ test.group('content probe', () => {
     const probe = await probeInspection(task, fetch)
     assert.equal(probe.contentType, 'nzb')
     assert.equal(probe.text, xml)
-    assert.lengthOf(fetched, task.segments.length + 1) // the first segment twice: once to sniff, once in the whole
+    // Every segment once: the first, decoded to sniff, is reused for the whole.
+    assert.deepEqual(fetched, task.segments.map((s) => s.id))
   })
 
   test('a 7z archive with encrypted headers is known from its tail, without bsdtar', async ({ assert }) => {