Read every page of the stat in a single snapshot and use the highest gen_id whose pages 0 to num_pages-1 are all present. Fall back if no generation is complete.

• Read with the INTERRUPT_ONLY yield policy and exhaust the cursor in one go (no getMores), so the read never abandons its snapshot.
• Assume gen_id = 0 for documents written before gen_ids existed.

See WRITING-11150, ANNEX II.