feat: column balancing for continuous section breaks

[jedrazb]

Problem

When a continuous section break switches from single-column to multi-column layout, Word balances the content across columns so they end at roughly the same height. Our current implementation fills column 0 completely before overflowing to column 1 (sequential fill).

Word behavior:

┌──────────────┬──────────────┐
│ Paragraph 1  │ Paragraph 3  │
│ Paragraph 2  │ Paragraph 4  │
└──────────────┴──────────────┘

Current behavior:

┌──────────────┬──────────────┐
│ Paragraph 1  │              │
│ Paragraph 2  │              │
│ Paragraph 3  │              │
│ Paragraph 4  │              │
└──────────────┴──────────────┘

Context

Multi-column layout rendering was added in PR #175. The paginator correctly handles column advancement when content overflows, but it doesn't balance columns for continuous section breaks. Balancing only applies to continuous breaks — nextPage section breaks fill columns sequentially (top-to-bottom, left-to-right) as we currently do.

Implementation Notes

Column balancing requires:

1. Detect when a continuous section break ends a multi-column section
2. Calculate total content height for that section's blocks
3. Binary search or iterative approach to find the optimal split point where all columns end at roughly the same Y position
4. Re-layout the section's content with the balanced split

Test file: examples/vite/public/continuous-columns-test.docx demonstrates the issue.

References

• ECMA-376 §17.6.4 (columns): balanced columns for continuous sections
• PR feat: wire multi-column layout rendering from section properties #175: multi-column layout implementation