test(ooxml): Sprint 16 - Document parsing edge cases

- Add is_monospace_font() helper tests for various font families
- Add bookmark filtering tests (_Hlk, _GoBack filtered; _Toc, _Ref kept)
- Add Document::blocks direct access and type checking tests
- Add Document::plain_text() with tables integration test
- Add Document::paragraphs() table flattening test
- Add NumberingRef parsing edge case (ilvl before numId)
- Add Paragraph::plain_text() with image alt text tests
- Add Hyperlink empty runs and whitespace-only edge cases
- Add textbox content parsing test
- Add table row/cell counts and multi-paragraph cell tests
- Add run monospace detection via font parsing test
- Add section break in blocks test
- 388 tests passing

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alanbld