Add line-level pagination with widow/orphan control

[JSv4]

Summary

Implements line-level paragraph splitting during pagination to prevent widows and orphans, addressing Issue #30. Paragraphs that exceed page height are now intelligently split at line boundaries rather than overflowing, with configurable minimum line constraints at page boundaries.

Key Changes

• Line measurement infrastructure: Added measureLines() method using the Range API and getClientRects() to precisely detect line boundaries within paragraph elements
• Paragraph splitting: Implemented splitParagraphAtLine() to split paragraphs at specific line boundaries using CSS clipping and margin adjustments
• Widow/orphan constraints: Added trySplitWithWidowOrphan() to enforce minimum line counts:
  • minOrphanLines (default: 2): minimum lines kept at bottom of page
  • minWidowLines (default: 2): minimum lines moved to top of next page
• Multi-page flow handling: Added flowSplitRemainder() and splitBlockAcrossPages() to handle remainders that span multiple pages
• Configuration options: Extended PaginationOptions with minOrphanLines and minWidowLines parameters
• Block metadata: Added widowControl property to MeasuredBlock interface, parsed from data-widow-control attribute (defaults to true per Word behavior)
• Pagination logic: Updated main pagination loop to attempt line-level splitting before moving entire blocks to new pages, respecting keepLines property

Implementation Details

• Line detection clusters character rectangles by vertical position to identify distinct lines
• Split points are validated against both orphan and widow constraints before applying
• Respects existing w:keepLines paragraph property to prevent splitting when specified
• Handles edge cases: single-line paragraphs, blocks taller than full page height, and remainder overflow
• Uses helper methods with callback parameters to manage state during complex multi-page splits

https://claude.ai/code/session_01PzsJ5ZDVcFnZ2tTo28bDBa