replace tui_markdown with a custom markdown renderer (#3396)

Also, simplify the streaming behavior.

This fixes a number of display issues with streaming markdown, and paves
the way for better markdown features (e.g. customizable styles, syntax
highlighting, markdown-aware wrapping).

Not currently supported:
- footnotes
- tables
- reference-style links

codex-rs/tui/src/lib.rs

@@ -45,6 +45,7 @@ pub mod insert_history;
 mod key_hint;
 pub mod live_wrap;
 mod markdown;
+mod markdown_render;
 mod markdown_stream;
 pub mod onboarding;
 mod pager_overlay;