Add clickable paragraph check box content controls - #1619

Summary

Add a public API for clickable check box content controls inside paragraphs. Paragraph.add_checkbox(checked=False) appends a control. Paragraph.checkboxes reads existing controls, and CheckBox.checked reads or updates their state. Changing the state also updates the visible glyph.

This uses Word's w:sdt content control with w14:checkbox metadata. The change has no dependency on the ps-python-docx fork or markdown-docx.

Validation

• Four saved-document tests pass for checked and unchecked controls, state changes, and table cells.
• Ruff checks pass for all changed files.
• The full upstream test suite has 56 failures in this Windows checkout, beginning with a fixture SHA-1 mismatch. The four added tests pass in that environment. The fork's full suite passed with 1,837 tests before release.