test(cos): qpdf can't check a prefixed file whose offsets include the prefix

qpdf counts every offset from the header in a file with bytes before it, so
it already reports the input of
`incremental_saves_of_a_file_whose_offsets_count_from_its_first_byte_keep_counting_so`
as damaged ("xref not found") and reconstructs it, before anything is
appended. The qpdf check there could never pass; the test's own reader checks
stay. The header-relative test keeps its qpdf check, which passes.

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

crates/cos/tests/writer.rs

@@ -453,5 +453,6 @@ fn incremental_saves_of_a_file_whose_offsets_count_from_its_first_byte_keep_coun
     assert!(back.repair_log().is_empty(), "{:?}", back.repair_log());
     assert_eq!(back.revisions().len(), 2);
     assert!(back.xref_entry_is_exact(1));
-    assert_qpdf_clean(&saved);
+    // No qpdf check here: qpdf counts every offset from the header in a file with bytes before
+    // it, so it already reconstructs this input before anything is appended.
 }