Markdown source/preview toolbar toggle changes position when file has Git changes

Does this issue occur when all extensions are disabled?: Yes (only the built-in Markdown and Git extensions are involved)

- VS Code Version: Code - OSS 1.134.0, commit `3e28a24456abff041d6ee809a33f24b37ab6a1a5`
- OS Version: macOS 26.6.1 (25G76), arm64

Steps to Reproduce:

1. Open a Git-tracked Markdown file in the text editor.
2. Modify the file so that it has working tree changes and the `Open Changes` editor-title action is visible.
3. Note the position of the `Open as Preview` action in the editor title.
4. Select `Open as Preview`.
5. Observe that `Open Changes` is now before `Reopen as source file`, so the source/preview toggle changes position.

Expected:

`Open as Preview` and `Reopen as source file` should occupy a stable position when switching between source and preview, including when the file has Git changes. This was the intent of #319723 and #319960.

Actual:

The toggle changes its position relative to `Open Changes`:

```text
Source editor: Open as Preview -> Open Changes
Preview editor: Open Changes -> Reopen as source file
```

This appears to happen because all three editor-title actions use the same `navigation@2` ordering:

- `markdown.reopenAsPreview`
- `markdown.reopenAsSource`
- `git.openChange`

When menu items have the same group and order, they are sorted by their original English titles. `Open as Preview` sorts before `Open Changes`, while `Reopen as source file` sorts after it. As a result, replacing the source action with the preview action swaps their relative positions.

Related: #319723, #319960