fix: pasted text not expanded correctly after CJK input

[xodmd45-ctrl]

Bug Description

When submitting a prompt with pasted content after typing CJK characters (Korean, Chinese, Japanese), the submitted message shows literal [Pasted text]\ instead of the actual pasted content.

Steps to Reproduce

1. Open opencode TUI
2. Type some Korean/Chinese/Japanese characters in the prompt
3. Paste some text after the CJK characters
4. Submit the prompt
5. Observe that the submitted message shows [Pasted text]\ literal instead of the actual pasted content

Root Cause

In \submitInner(), extmark \start/\�nd\ are display-width offsets (visual columns), not JS string indices. \slice()\ expects character indices. For CJK characters (visual width 2, string length 1), these diverge, causing \slice()\ to cut at wrong positions.

Environment

• OS: Windows 11
• Terminal: Windows Terminal
• opencode version: dev branch (latest)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [Pasted ~N lines] is partially submitted when prompt contains CJK or other multi-width characters #17032: Same root cause — [Pasted ~N lines] placeholder partially submitted when prompt contains CJK/multi-width characters due to visual-width offsets vs JS string indices mismatch
• Summarized pasted text is corrupted around wide characters in TUI prompt #29707: Same symptom — pasted text corrupted around wide characters in TUI prompt (also on Windows 11 with CJK input)
• [Bug] The "paste summary" causes the content to be disordered after pasting. #25854: Related — paste summary causes content disorder after pasting

If your issue is the same as one of the above, please consider closing this one and adding context there instead.

[simonklee]

#29710