- Fix 8 tests that incorrectly passed ParentTree dict directly instead of
wrapping it in a StructTreeRoot-like structure with /ParentTree key
- Fix process_nums_array() to preserve null entries as ObjRef { object: 0 }
instead of filtering them out, ensuring orphan MCIDs are correctly reported
- Add verification note for ParentTree-based MCID-to-StructElem resolver
References: pdftract-57o4, plan 7.1 line 2550 (MCID-to-StructElem mapping)
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