Document scopedcustomelementregistry and its HTMLElement reflection

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry

### What information is incomplete?

The HTML Standard is adding the `scopedcustomelementregistry` global content attribute and the reflecting `HTMLElement.scopedCustomElementRegistry` IDL attribute.

The content attribute affects HTML parser creation: the element and its parsed subtree begin with a null custom element registry so a scoped `CustomElementRegistry` can claim the subtree later through `initialize()`. Changing the attribute after element creation does not change the assigned registry. The IDL attribute reflects as a `DOMString` for future extensibility.

### What should be documented?

- The `scopedcustomelementregistry` global attribute.
- `HTMLElement.scopedCustomElementRegistry` string reflection.
- Parser-only behavior and the relationship to `CustomElementRegistry.initialize()`.
- Serialization behavior at custom element registry boundaries.
- Browser compatibility once implementations adopt the final specification.

### References

- HTML specification PR: https://github.com/whatwg/html/pull/12000
- DOM infrastructure PR: https://github.com/whatwg/dom/pull/1500
- Web Platform Tests PR: https://github.com/web-platform-tests/wpt/pull/62206

Start with the MDN CustomElementRegistry page and compare the requested behavior with the linked HTML, DOM, and Web Platform Tests pull requests. Document the global attribute, HTMLElement.scopedCustomElementRegistry reflection, parser-only behavior, initialize() relationship, and serialization boundaries. Finish by adding browser compatibility information once implementations adopt the final specification.

Written by the indexing model from the issue text.