Add Word horizontal rules to paragraphs - #1621

Summary

Add Paragraph.add_horizontal_rule() for Word's built-in horizontal-line representation. Calling it on an empty paragraph applies a bottom paragraph border with a simple line. It preserves existing paragraph properties and is idempotent.

Microsoft documents that Word's three-character horizontal-line shortcut creates an empty paragraph with a bottom border: https://support.microsoft.com/en-us/word/insert-a-horizontal-line

Validation

• Two saved-document tests pass against upstream master. They check the serialized border and preserved paragraph properties.
• Ruff checks pass for all changed files.
• The fork's full test suite passed with 1,839 tests before its 1.3.10 release.