Guarantee uniqueness in `RelationshipTarget`

## What problem does this solve or what need does it fill?

Certain algorithms require a guarantee that the entities in a `RelationshipTarget` are unique for performant and correct behavior. Specifically this is required for transform propagation, which I am looking to have support user-defined relationships in https://github.com/bevyengine/bevy/issues/25468.

## What solution would you like?

`RelationshipTarget::Collection::SourceIter` should be required to implement `EntitySetIterator`. With this change `Vec` could not be used as `RelationshipTarget::Collection` anymore as safe user code edit it to contain duplicates, causing undefined behavior. This is currently the case for `Children`, and is already causing undefined behavior as seen here https://github.com/bevyengine/bevy/issues/25349. We would need a Vec that prevents duplicates, or use other collections such as `EntityIndexSet`, see https://github.com/bevyengine/bevy/pull/23296.

## What alternative(s) have you considered?

We could add a `UniqueRelationshipTarget` supertrait that is implemented for every `RelationshipTarget` which fulfills the requirement above. This way non-unique relationships would still be possible, though I am not aware of a use case for those.

Start with RelationshipTarget::Collection::SourceIter and the EntitySetIterator requirement, then inspect Children and the referenced EntityIndexSet work. Compare the proposed required iterator with the UniqueRelationshipTarget alternative and trace the linked undefined-behavior issues. Done means relationship targets cannot be safely edited into duplicate-containing collections, while the intended transform-propagation use case remains supported.

Written by the indexing model from the issue text.