Renders the per-file added/removed line counts (already tracked per
buffer by the multibuffer) in each diff multibuffer header, matching the
overall stat shown in the branch diff toolbar.

### Before
<img width="1624" height="1061" alt="Screenshot 2026-07-02 at 11 50 42
AM"
src="https://github.com/user-attachments/assets/3e46a90c-a4e1-4b72-8ec9-80931a7e4a9a"
/>

### After
<img width="1624" height="1061" alt="Screenshot 2026-07-02 at 12 03 58
PM"
src="https://github.com/user-attachments/assets/25900790-82f5-44e9-9a3b-731a3090f23e"
/>


Release Notes:

- Improved diff multibuffer headers to show per-file added/removed line
counts

---------

Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>