`Spawn<B>` now does an unnecssary archetype move / split insert

As of https://github.com/bevyengine/bevy/pull/20772, we are now unnecessarily splitting the `spawn((bundle, relationship))` into `spawn(relationship).insert(bundle)`, resulting in an unnecessary archetype move and redundant bookkeeping. This needs to be fixed, as `Spawn` is a high-traffic api.

This points to missing tools in our toolkit!

Start by reviewing the changes in pull request 20772 and tracing the Spawn path for spawn((bundle, relationship)). Confirm where the operation becomes spawn(relationship).insert(bundle), then verify that the completed change avoids the unnecessary archetype move and redundant bookkeeping.

Written by the indexing model from the issue text.