[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.