fix: preserve section boundaries in multiline headers

[peaceroad]

Summary

• retain the logical row token after multiline-row compaction
• restore the missing thead_close token for multiline headers
• add an HTML regression fixture
• add direct token-order and open/close balance assertions

When a multiline header is compacted, trToken continues to refer to the discarded final physical-row token. The following separator therefore places the group-closing flag on that discarded token instead of the retained logical row. As a result, the rendered table can open tbody before closing thead.

Reassigning trToken to the retained token after compaction preserves the section boundary.

Validation

• the regression fixture confirms that thead_close occurs before tbody_open
• thead, tbody, and tr open/close token counts remain balanced
• existing multiline, rowspan, headerless, and multibody fixtures pass
• all 56 tests pass with markdown-it 13.0.2
• coverage remains at 100%
• lint passes

Fixes #69.

This pull request was prepared with assistance from GitHub Copilot (GPT).