## Summary
- replace the word part enum with a simple `is_word_separator` helper
- keep word-boundary logic aligned with the helper and punctuation-aware
behavior
- extend forward/backward deletion tests to cover whitespace around
separators
## Testing
- just fix -p codex-tui
- cargo test -p codex-tui
------
https://chatgpt.com/codex/tasks/task_i_68f91c71d838832ca2a3c4f0ec1b55d4