Deduplicate TUI modifier handling and terminal color caching

[copyberry]

Deduplicate TUI modifier handling and terminal color caching

What changed

• Reuse custom_terminal::ModifierDiff when writing history spans.
• Share the terminal default-color probe and cache implementation between Unix and Windows.

Both changes remove duplicate implementations while preserving existing behavior.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	🔄 Running since 2026-10-13T04:42:15.386397Z	8377d5e	PR opened

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