Expose configurable repeating table header rows - #1617

Expose row.repeat_as_header as a tri-state public property. True enables the direct setting, False explicitly disables it, and None removes it. New rows retain their existing default behavior.

This change preserves cell content, merged cells, row height, and unrelated properties. It only adds the repeating-header control. Word repeats a contiguous group of marked leading rows. Marking a later row does not make it repeat.

Related to #322 and the older proposal in #226. This draft targets the current row/XML architecture, adds explicit false and clear semantics, and includes tests and documentation. It contains no fork packaging or other feature commits.

Validation:

• 1,623 upstream unit tests and 651 acceptance scenarios passed.
• Lint and type checks passed.
• A Word-authored fixture covers leading headers and unmarked rows.
• A four-page Word PDF repeats both leading headers on every page and shows a marked non-leading row only once.