This was mostly written by codex under heavy guidance via test cases
drawn from logged session data and fuzzing. It also uncovered some bugs
in tui_markdown, which will in some cases split a list marker from the
list item content. We're not addressing those bugs for now.
Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.
We wait until we have an entire newline, then format it with markdown and stream in to the UI. This reduces time to first token but is the right thing to do with our current rendering model IMO. Also lets us add word wrapping!
