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/diff_render.rs

@@ -67,7 +67,7 @@ impl From<DiffSummary> for Box<dyn Renderable> {
             rows.push(Box::new(path));
             rows.push(Box::new(RtLine::from("")));
             rows.push(Box::new(InsetRenderable::new(
-                row.change,
+                Box::new(row.change) as Box<dyn Renderable>,
                 Insets::tlbr(0, 2, 0, 0),
             )));
         }