IME composition causes window flickering/shaking on Windows

## Description

When typing CJK characters (Chinese/Japanese/Korean) using Windows IME in the Copilot CLI, the terminal window visibly flickers/shakes on each keystroke during IME composition.

## Steps to Reproduce

1. Open Copilot CLI in Windows Terminal
2. Switch to a CJK input method (e.g., Microsoft Bopomofo / Pinyin)
3. Start typing Chinese characters
4. Observe the terminal window shaking/flickering as the IME candidate window appears

## Expected Behavior

The TUI should not re-render/redraw the entire frame during IME composition events. The input area should remain stable while the user is composing characters.

## Actual Behavior

Every IME composition event triggers a full TUI re-render, causing visible window flickering that makes the CLI uncomfortable to use with CJK input methods.

## Environment

- OS: Windows 11
- Terminal: Windows Terminal
- PowerShell: 7.6.1
- Copilot CLI: 1.0.40-0
- IME: Microsoft Bopomofo (Traditional Chinese)
- Code Page: 65001 (UTF-8)

## Impact

This significantly degrades UX for CJK users and discourages use of the CLI in non-English languages.

首先，在 Windows Terminal 中使用 Microsoft Bopomofo 或 Pinyin 输入法复现 Copilot CLI 中的问题。跟踪 IME 组合事件如何到达 TUI 重绘路径，并验证组合不再触发整个帧的重新渲染。完成的标准是 CJK 组合在视觉上保持稳定，同时普通输入继续正常工作。

由索引模型根据 Issue 内容生成。