Implement serialize_page_text() function that iterates blocks in
reading order, filters by block-kind (Header/Footer/Watermark),
joins block texts per kind-specific rules, and separates blocks
with \n\n.
- Add new text.rs module with TextOptions and serialize_page_text()
- Paragraph/Heading/Caption/Quote: use pre-computed block text
- List/Code: preserve newlines from pre-computed text
- Figure: emit empty string
- Empty blocks omitted (no spurious newlines)
- Headers/footers/watermarks excluded by default, configurable
Closes: pdftract-529te
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
