Add TUI building blocks for inline async question editing

[copyberry]

Add TUI building blocks for inline async question editing

What changed

• Introduce an async-question editor component with per-question drafts, navigation, replay deduplication, and submit or queue handling using the shared composer.
• Add bounded AnsweredQuestion framing that truncates question text at a UTF-8 boundary and flattens line breaks before prepending it to an answer.
• Add prompt_stack_back and skip_question keybinding actions and the tui.question_esc_back setting. Normalize Ctrl+] and Ctrl+5 for key matching and conflict detection, and let default question shortcuts yield to explicit bindings.
• Flush buffered typing in both the main composer and the covering view so background input cannot keep the draw loop waiting.

The async-question component is not yet connected to the TUI event flow.

Testing

Add regression coverage for bounded Unicode question framing, question-shortcut conflicts, and paste-buffer flushing in background and covering editors.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

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

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}