[BUG] Japanese (CJK) characters garbled during streaming output

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Japanese (CJK multi-byte) characters in the assistant's streaming output are intermittently garbled/corrupted. Characters appear to be split at token boundaries during streaming, resulting in broken/missing characters in the rendered text.

Examples of corrupted output observed:

• 50チケッ��なら (expected: 50チケットなら)
• 関連チケットと Scope外に反映しま�� (missing final character)
• PR #356 ��修正に戻りますか？ (corrupted character mid-sentence)

How to Reproduce

1. Set locale to ja_JP.UTF-8
2. Have a conversation in Japanese where Claude generates multi-paragraph responses
3. Observe that some CJK characters are garbled/missing in the streamed output

The issue occurs randomly during streaming. It is not specific to any particular character — different characters are corrupted in different responses.

Environment

• Claude Code version: 2.1.94
• OS: macOS (Darwin 25.3.0)
• Terminal: Ghostty (xterm-ghostty), also reproduced in VS Code integrated terminal and via cmux
• Shell: zsh
• Locale: ja_JP.UTF-8

Additional Context

• This is NOT the spinner/status line garbling reported in #42177 — this affects the main assistant output text
• The issue reproduces across multiple terminal emulators (Ghostty, VS Code terminal), ruling out terminal-specific rendering bugs
• The issue also reproduces when using cmux (terminal multiplexer), but is not caused by it since it reproduces without cmux as well