Vue3: `mutationobserver-shim`

[lreading]

Describe what problem your feature request solves:

The mutationobserver-shim may no longer be required after the vue3 migration.
This should be explored after the vue-compat layer is removed.

Describe the solution you'd like:

• Verify if it's still needed after compat / Vue 2 dependencies are removed.
• Remove the import and dependency if modern Vue 3/runtime behavior no longer needs it.

Declaration:

By submitting this issue you have:

• read the contribution guide and agree to the Code of Conduct
• not used agentic or generative AI in creating this feature request

Additional context:

This could be a no-op - but it deserves research and understanding. This work was intentionally deferred during the initial vue3-compat work.