fix: move the structure entry's pages to the convention's total + pagination #67

Found while reviewing ICIJ/datashare#2299, which makes Java a structure producer.

The two sides do not agree on where a paginated entry keeps its page count. StructureManifestEntry nests everything under pages (pages: {type, total, byteRanges}, pinned in tests/test_objects.py:108-113), while the storage convention and now Java put them in the entry itself:

"structure": {
  "status": "complete",
  "taskInput": {...},
  "total": 3,
  "pagination": {"type": "byteRanges", "ranges": [[0,1],[1,2],[2,3]]}
}

Neither side can read the other's page count today. Java no longer destroys the fields it does not model (it merges the manifest at tree level), so this is a read/compatibility gap rather than data loss, but a consumer cannot serve a docling-produced structure through the Java path until the shapes match.

Fix: move pages to the convention's entry-level total + pagination, where pagination carries type and, for the byteRanges scheme, ranges.