Add Unicode grapheme handling guidance

[akrantz]

Add guidance to CONTRIBUTING.md and .claude/rules/ about correct grapheme-aware text handling: use string.GetColumns() for width measurement, iterate by grapheme with GraphemeHelper.GetGraphemes() for rendering, and use AddStr instead of AddRune for grapheme clusters.

This documents the patterns that prevent the bugs fixed in #4798 and #4800 from recurring.

Fixes

• Add Unicode grapheme handling guidance to CONTRIBUTING.md #4802

Proposed Changes/Todos

• Add "Unicode and Grapheme Handling" subsection to Coding Conventions in CONTRIBUTING.md
• Add corresponding entries to "What NOT to Do" section
• Add .claude/rules/unicode-graphemes.md for AI agent guidance

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

Thank you!!!!