Fixes split-view rendering for long CJK lines, especially untracked files with Mandarin content, where mouse-wheel repainting could let wide characters spill past the right split pane into the left side.

Changes:

• Measure diff code and UI text by terminal cell width instead of UTF-16 string length.
• Slice, pad, wrap, and horizontally scroll styled diff spans without splitting fullwidth characters.
• Add Mandarin-only long-line regression coverage for mouse-wheel split-view repainting.
• Add Japanese and Korean unit coverage for CJK width and clipping behavior.

Before

After