[DOM] Fix Fragment compareDocumentPosition(document) TypeError (#37579)

## Summary

Fixes #37578.

`FragmentInstance.compareDocumentPosition(document)` threw:

```
TypeError: Cannot read properties of null (reading 'documentElement')
```

because `Document.ownerDocument` is `null`, and the `CONTAINS`
fiber-validation fallback read `ownerDocument.documentElement`
unconditionally. `document.body` / `document.documentElement` already
worked.

This treats a `Document` node as its own owner document, matching
`Node.compareDocumentPosition(document)`.

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Co-authored-by: Cursor <cursoragent@cursor.com>