#### Context

Switches `TextDiffView` from using `Editor` directly to
`SplittableEditor`, enabling side-by-side diff view support for "Diff
Clipboard with Selection". The diff view now respects the user's
`diff_view_style` setting.

Split out from #51457. This PR contains only the `SplittableEditor`
wiring. The multibuffer coordinate fix for non-singleton editors will
follow in a separate PR.

Closes #50912 (partially)

#### How to Review

Small PR — all changes are in `crates/git_ui/src/text_diff_view.rs`.
Focus on:
- `new()`: `SplittableEditor::new` replaces `Editor::for_multibuffer`,
editor-specific setup goes through `rhs_editor()`
- Item trait delegation: `act_as_type`, `for_each_project_item`,
`set_nav_history` updated for `SplittableEditor`
- Tests: pinned `DiffViewStyle::Unified` and assertions go through
`rhs_editor()`

#### Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

#### Video : 
[Screencast from 2026-03-19
23-11-36.webm](https://github.com/user-attachments/assets/c5a2381d-238d-43ef-ac6f-9994996c0c69)


#### Release Notes:

- Improved "Diff Clipboard with Selection" to support side-by-side diff
view style.