dynamic width for line numbers in diffs (#4664)

instead of always reserving 6 spaces for the line number and gutter, we
now dynamically adjust to the width of the longest number.

<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 00 AM"
src="https://github.com/user-attachments/assets/5f18eae6-7c85-48fc-9a41-31978ae71a62"
/>
<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 21 AM"
src="https://github.com/user-attachments/assets/9009297d-7690-42b9-ae42-9566b3fea86c"
/>
<img width="871" height="616" alt="Screenshot 2025-10-03 at 8 21 57 AM"
src="https://github.com/user-attachments/assets/669096fd-dddc-407e-bae8-d0c6626fa0bc"
/>

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

@@ -4,6 +4,9 @@ use ratatui::text::Line;
 use ratatui::widgets::Paragraph;
 use ratatui::widgets::WidgetRef;
 
+use crate::render::Insets;
+use crate::render::RectExt as _;
+
 pub trait Renderable {
     fn render(&self, area: Rect, buf: &mut Buffer);
     fn desired_height(&self, width: u16) -> u16;
@@ -100,3 +103,26 @@ impl ColumnRenderable {
         }
     }
 }
+
+pub struct InsetRenderable {
+    child: Box<dyn Renderable>,
+    insets: Insets,
+}
+
+impl Renderable for InsetRenderable {
+    fn render(&self, area: Rect, buf: &mut Buffer) {
+        self.child.render(area.inset(self.insets), buf);
+    }
+    fn desired_height(&self, width: u16) -> u16 {
+        self.child
+            .desired_height(width - self.insets.left - self.insets.right)
+            + self.insets.top
+            + self.insets.bottom
+    }
+}
+
+impl InsetRenderable {
+    pub fn new(child: Box<dyn Renderable>, insets: Insets) -> Self {
+        Self { child, insets }
+    }
+}