Par2 merge: a set file matching several releases nearby is ambiguous

131 uploads of 1 GiB test files ("kantodoNNN.bin", a.b.test) share their
first 16 KiB and size, so all matched one set's single file and became one
release of 1,284 files. A par2 file that matches more than one release in
the anchor's group and time window is left out; a plain repost of the same
file stays apart too.

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

tests/functional/par2_merger.spec.ts

@@ -62,4 +62,25 @@ test.group('Par2Merger', (group) => {
     // Again: nothing new.
     assert.deepEqual(await new Par2Merger().round().then((r) => r.merges), 0)
   })
+
+  test('a set file that matches several releases nearby is ambiguous: not merged', async ({ assert }) => {
+    const at = new Date('2022-07-08T03:46:00Z')
+    const ids: number[] = []
+    for (const key of ['aa000001', 'aa000002', 'aa000003']) {
+      const [row] = await db
+        .table('releases')
+        .insert({ release_key: key.padEnd(40, '0'), identity_key: key.padEnd(40, '0'), name: key, source: 'stem', groups: '[]', group_id: 1, files: 1, posted_at: at, readable: false })
+        .returning('id')
+      ids.push(Number(row.id))
+    }
+    // Two test uploads whose 1 GiB files start alike, and a set describing one such file, held by the third.
+    await db.table('file_hashes').multiInsert([
+      { candidate_id: 2001, release_id: ids[0], hash16k: hash('f'), size: 1073741824 },
+      { candidate_id: 2002, release_id: ids[1], hash16k: hash('f'), size: 1073741824 },
+    ])
+    await db.table('par2_files').insert({ set_id: hash('9'), file_id: hash('8'), hash16k: hash('f'), length: 1073741824, name: 'kantibanito1.bin' })
+    await db.table('par2_sets').insert({ set_id: hash('9'), release_id: ids[2], files: 1 })
+    assert.equal((await new Par2Merger().round()).merges, 0)
+    assert.equal((await db.from('releases').whereIn('id', ids).select('id')).length, 3)
+  })
 })