Implements the StructTree parser (Phase 7.1.1) with:
- Depth-first walker over /StructTreeRoot via /K array
- Support for all four /K entry types: StructElem, MCID, MCR, OBJR
- /RoleMap resolution with chain handling and cycle detection
- /Lang inheritance through the structure tree
- /ActualText inheritance (applies to all descendant content)
- Public API: StructureType, StructElemNode, StructTreeRoot, RoleMap, Kid
Acceptance criteria:
- PASS: All four /K element kinds handled without crashing
- PASS: /RoleMap chains resolve to standard type or NonStruct
- PASS: /Lang and /ActualText inherit correctly down tree
- PASS: Unit tests for Word RoleMap (Heading1 -> H1)
- PASS: Unit tests for nested /Lang and /ActualText scope
- PASS: Public type StructElemNode documented in core crate
References:
- Plan section 7.1 StructTree Exploitation (lines 2547-2549, 2552-2553)
- PDF 1.7 spec 14.7.4 (Structure Tree) and 14.8.4 (Standard Structure Types)
Co-Authored-By: Claude Code <noreply@anthropic.com>
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