Add a boundary alignment command

[kisaraki]

Summary

• add Boundary Alignment as the final Edit-menu command
• prompt for a positive column, defaulting to 80
• split text after the selected character boundary and carry overflow into the following line through EOF
• count Unicode characters and preserve CRLF/final-newline behavior

Validation

• unit tests cover overflow merging, Unicode, CRLF, final newline, and unchanged short lines
• cargo test --all-features --all-targets on Windows and WSL/Ubuntu
• cargo clippy --workspace --all-features --all-targets -- --no-deps --deny warnings on Windows and WSL/Ubuntu

Contribution ownership

This contribution is entirely my own work. It is not created on behalf of, or connected with, any organization or company. I agree to contribute it under the repository's MIT License and to the repository's applicable contribution terms.