tui: refactor ChatWidget and BottomPane to use Renderables (#5565)

- introduce RenderableItem to support both owned and borrowed children
in composite Renderables
- refactor some of our gnarlier manual layouts, BottomPane and
ChatWidget, to use ColumnRenderable
- Renderable and friends now handle cursor_pos()

codex-rs/tui/src/exec_cell/render.rs

@@ -19,9 +19,6 @@ use itertools::Itertools;
 use ratatui::prelude::*;
 use ratatui::style::Modifier;
 use ratatui::style::Stylize;
-use ratatui::widgets::Paragraph;
-use ratatui::widgets::WidgetRef;
-use ratatui::widgets::Wrap;
 use textwrap::WordSplitter;
 use unicode_width::UnicodeWidthStr;
 
@@ -205,31 +202,6 @@ impl HistoryCell for ExecCell {
     }
 }
 
-impl WidgetRef for &ExecCell {
-    fn render_ref(&self, area: Rect, buf: &mut Buffer) {
-        if area.height == 0 {
-            return;
-        }
-        let content_area = Rect {
-            x: area.x,
-            y: area.y,
-            width: area.width,
-            height: area.height,
-        };
-        let lines = self.display_lines(area.width);
-        let max_rows = area.height as usize;
-        let rendered = if lines.len() > max_rows {
-            lines[lines.len() - max_rows..].to_vec()
-        } else {
-            lines
-        };
-
-        Paragraph::new(Text::from(rendered))
-            .wrap(Wrap { trim: false })
-            .render(content_area, buf);
-    }
-}
-
 impl ExecCell {
     fn exploring_display_lines(&self, width: u16) -> Vec<Line<'static>> {
         let mut out: Vec<Line<'static>> = Vec::new();