fix: diff_buffers clear-to-end when deleting wide graphemes

[MomentDerek]

fix #4870 #4717

• Fix the chat composer “phantom space” bug that appeared when backspacing CJK (and other double-width) characters after the composer got a uniform background in 43b63cc….
• Pull diff_buffers’s clear-to-end logic forward to iterate by display width, so wide graphemes are counted correctly when computing the trailing column.
• Keep modifier-aware detection so styled cells are still flushed, and add a regression test (diff_buffers_clear_to_end_starts_after_wide_char) that covers the CJK deletion scenario.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[MomentDerek]

I have read the CLA Document and I hereby sign the CLA

[joshka-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

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[MomentDerek]

@joshka-oai Due to no remaining space in GitHub Action, one of the CI services has stopped running, can you help me restart it?

[joshka-oai]

Force pushed a rebase on main and a few small tweaks to make it easier to read / understand this code going forward.

Tested that this fixes the problems rendering Chinese (#4717), Korean (#4898, #4870), and Japanese (#3260) characters correctly.

[joshka-oai]

LGTM - thanks for the PR