[BUG] Terminal input cursor position drifts during Japanese IME (full-width) input

## Summary
When typing Japanese text via IME (full-width characters) in the terminal
prompt, the displayed cursor position gradually drifts away from the actual
input position while composing. Pressing Enter re-renders the line and the
display snaps back to the correct position.

## Environment
- Claude Code CLI: 2.1.241

## Steps to reproduce
1. Open Claude Code in a terminal
2. Switch IME to Japanese input, type full-width characters
3. Observe the cursor rendering position vs. the actual character position
while composing (before pressing Enter)

## Expected behavior
The displayed cursor position should stay aligned with the actual input
position while composing full-width text.

## Actual behavior
- Does NOT reproduce with half-width (ASCII) input — only full-width IME
composition triggers the drift.
- No functional impact observed (the submitted text itself is correct once
Enter is pressed); it's purely a visual/rendering offset during composition.
- Not found in CHANGELOG entries around v2.1.239–2.1.241.

Start by reproducing the cursor drift in the Claude Code terminal prompt with Japanese full-width IME input, comparing the composing display with the position after pressing Enter. The issue is done when the displayed cursor remains aligned during composition while ASCII input and submitted text continue to work.

Rédigé par le modèle d'indexation à partir du texte de l'issue.