fix: preserve Rust decorator attribute color

[kyerpotts]

Summary

Verification

• Headless test suite
• Direct colorscheme smoke test
• StyLua and git diff --check
• Manual visual inspection, or not applicable

Visual changes

Checklist

• My PR title follows Conventional Commits.
• This change is focused and includes no unrelated cleanup.
• I added or updated tests and documentation where needed.
• I verified upstream highlight groups for integration changes.
• I read and understand all submitted changes, including any AI-assisted work.