Rebuild: fragments only for files an edge cut; lone random articles are per-article

Measured on two neighbouring boneless ranges (merging their fragments):
- 99.8% of fragment files were single articles of a random-named
  'file' of >100 parts: a random subject per article, which never
  completes from headers. They now count as per-article.
- A file is kept as a fragment when its first part seen is above 1 and
  comes within the start window (the idle window: uploads in busy groups
  spread over minutes, and a 50k-article edge zone is ~30 s of boneless),
  or when it was still open at the range's end with later parts missing.
  Anything else missing parts is incomplete and only counted.
Per range: 70,308 → 130 fragment files; merged, 47 of the 48 files met
complete (57,558 segments, ~8% of the range's kept segments).

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

tests/functional/batch_store.spec.ts

@@ -42,7 +42,7 @@ test.group('BatchStore', (group) => {
       ...post(100_000, 'Some.Show.S01E01.1080p.WEB.part1.rar', 2, 'Some.Show.S01E01 [1/2]'),
       ...post(100_010, 'Some.Show.S01E01.1080p.WEB.par2', 1, 'Some.Show.S01E01 [2/2]'),
       ...post(100_020, 'Hs7dKq2LmZ9xWv3RtYp1Qa.part01.rar', 3, '[1/1]', 'ngPost'),
-      ...post(10, 'Edge.Cut.2020.part1.rar', 3, 'Edge.Cut [1/1]').slice(0, 1),
+      ...post(10, 'Edge.Cut.2020.part1.rar', 3, 'Edge.Cut [1/1]').slice(1),
     ], 500_000)
     const first = ix.take()
     const stored = await BatchStore.store(rangeId, nodeId, first)