feat(pagination): Line-level pagination with paragraph splitting

[JSv4]

Summary

Currently the pagination engine operates at the block (paragraph) level. If a paragraph is taller than a page, it overflows and gets clipped. This creates a poor user experience for documents with long paragraphs.

Proposed Solution

Implement line-level measurement and splitting:

1. Line measurement: Use getClientRects() or range-based measurement to identify individual lines within a paragraph
2. Paragraph splitting: When a paragraph doesn't fit, split it at a line boundary
3. Orphan/widow control: Ensure at least N lines remain on each page (configurable)

Implementation Approach

interface MeasuredLine {
  startOffset: number;
  endOffset: number;
  height: number;
}

function measureLines(element: HTMLElement): MeasuredLine[] {
  // Use Range API to measure each line
  // Handle inline elements, images, etc.
}

function splitParagraph(element: HTMLElement, maxHeight: number): [HTMLElement, HTMLElement] {
  // Clone and split at line boundary
}

Challenges

• Complex inline content (images, links spanning lines)
• Maintaining semantic correctness when splitting
• Performance for large documents
• RTL and vertical text layouts

Acceptance Criteria

• Paragraphs split cleanly at line boundaries
• Minimum 2 lines on each page (no orphans/widows)
• Inline elements handled correctly
• No visual artifacts at split points