syntax-highlight bash lines (#3142)

i'm not yet convinced i have the best heuristics for what to highlight,
but this feels like a useful step towards something a bit easier to
read, esp. when the model is producing large commands.

<img width="669" height="589" alt="Screenshot 2025-09-03 at 8 21 56 PM"
src="https://github.com/user-attachments/assets/b9cbcc43-80e8-4d41-93c8-daa74b84b331"
/>

also a fairly significant refactor of our line wrapping logic.

codex-rs/tui/src/backtrack_helpers.rs

@@ -12,7 +12,7 @@ pub(crate) fn highlight_range_for_nth_last_user(
 /// Compute the wrapped display-line offset before `header_idx`, for a given width.
 pub(crate) fn wrapped_offset_before(lines: &[Line<'_>], header_idx: usize, width: u16) -> usize {
     let before = &lines[0..header_idx];
-    crate::insert_history::word_wrap_lines(before, width).len()
+    crate::wrapping::word_wrap_lines(before, width as usize).len()
 }
 
 /// Find the header index for the Nth last user message in the transcript.