Fix or quarantine IME composition positioning in terminal

[kyroskoh]

Context

Known TODO: IME composition position is wrong in the terminal. zh locale is shipped, so CJK users hit this.

Goal

Correct IME composition caret/candidate positioning, or quarantine/document a reliable workaround until fixed.

Acceptance criteria

• Reproduce on Windows (and ideally macOS) with CJK IME
• Fix composition positioning for xterm, or ship a documented temporary mitigation
• Resolve/update TODO in src/renderer/xterm.css
• Manual QA checklist for zh (and optionally ja/ko) input in SSH session

References

• src/renderer/xterm.css (~IME TODO)