Add Ctrl+P/Ctrl+N navigation across composer and selection popups

[mayphus]

Summary

• handle Ctrl+P and Ctrl+N chords in the composer textarea so vertical movement works across terminals
• map the fallback C0 control bytes for those chords to the same navigation paths in both the composer and generic list selection popups
• wire the same shortcuts into list selection popups so Emacs-style navigation works consistently
• extend textarea and list selection tests to cover both the modifier chords and C0 fallbacks

Testing

• cargo test -p codex-tui
• just fmt
• just fix -p codex-tui

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[mayphus]

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

[etraut-openai]

Thanks for the contribution, and apologies for the slow response. We've received many PRs, and we don't have the bandwidth on the codex team to review all of them.

We've updated our contribution guidelines to clarify that we're currently accepting contributions for bugs and security fixes, but we're not generally accepting new features at this time. We need to make sure that all new features compose well with both existing and upcoming features and fit into our roadmap. If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.