Lone articles under scrambled names are per-article; inactive groups get no corpus fill

One article of a multi-part file under a name like "nYDeXeF49dieZ1J7flK-JFzZ3j.ZEDsr" or
"bslXlwgPiqZadhRa" (a random subject and poster per article, with fake counters) was a release:
looksRandom misses these names for their separators. 160k of alt.binaries.storage's 162k
non-Bob releases, and about 60% of all single-article releases in a 2% sample.

looks_scrambled / isScrambledFile: at least half the stem's letters and digits in tokens of 5+
that switch case or letter/digit 3 times or more (93% of the storage spam). Used for lone
articles given up before the block's end only; at the end the strict looksRandom stays, as the
rest of such a file can be in the next block. The namer's re-index (partialFiles) does the same.

The prefetcher's corpus fill skips groups that aren't active.

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

tests/unit/edge/range_indexer.spec.ts

@@ -105,6 +105,20 @@ test.group('edge / RangeIndexer', () => {
     assert.deepEqual(batch.fragments, [])
     assert.deepEqual(batch.counts.map((c) => [c.rule, c.articles, c.files]), [['per-article', 1, 0]])
   })
+
+  test('partialFiles: one article of a multi-part file under a scrambled name is per-article before the end, a partial release at it', ({ assert }) => {
+    const indexer = new RangeIndexer({ group: GROUP, start: 100, end: 1_000_000, rules, idleChunks: 1, edgeArticles: 0, partialFiles: true })
+    // Part 8 of 67, a random poster's: given up mid-range.
+    indexer.add(post(100, 'nYDeXeF49dieZ1J7flK-JFzZ3j.ZEDsr', 67, { poster: 'guAym5TqSlp SCTLKQ0R <8nGc3Rk@OrjrYqFkp.z>' }).slice(7, 8), 200)
+    indexer.add([], 300)
+    indexer.add([], 400)
+    // The same shape still open when the block ends: the rest can be in the next one.
+    indexer.add(post(999_990, 'bslXlwgPiqZadhRa', 5).slice(1, 2), 1_000_001)
+    indexer.finish()
+    const batch = indexer.take()
+    assert.deepEqual(batch.releases.map((r) => [r.name, r.partial]), [['bslXlwgPiqZadhRa', true]])
+    assert.deepEqual(batch.counts.map((c) => [c.rule, c.articles, c.files]), [['per-article', 1, 0]])
+  })
 })
 
 test.group('edge / taking over a range', () => {