Some files carry bytes before `%PDF-`, and their offsets count from the header rather than from
the first byte. We read those, retrying a section relative to the header when the absolute
offset misses, but the incremental writer then appended a section whose offsets and `startxref`
were absolute while its `/Prev` kept the header-relative value it was given. qpdf reported "xref
not found" and reconstructed the file, so a save that reported success left a damaged-looking
update behind (#2364, found with KillerPDF's reserve3.pdf and a Subject edit).
The reader now records on each `Revision` whether it was found counting from the header, and the
writer counts the new section the same way as the one its `/Prev` points to. A file whose
offsets already include the prefix keeps absolute offsets, which the second test pins so the fix
cannot overcorrect.
Seen red with the writer change reverted: the appended section's `startxref` missed its `xref`.
Fixesstorytold/pdfcraft#2364
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DsT1y8xBvkXeBxEUQqdDtj
