Fix Chinese IME marked text handling

[Pluviobyte]

Summary

• skip SwiftUI/AppKit text synchronization while the editor has marked text from an input method
• avoid reapplying highlighting or restoring selection during IME composition
• allow Chinese IME pre-edit text and candidate selection to complete before updating the bound script text

Fixes #34.

Validation

• Manually verified on macOS with Sogou Pinyin: typed nihao and committed it as 你好 in the Textream editor
• Manually verified composition stays visible before committing ceshi to 测试
• git diff --check