Scene prefab

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

Ability to use `Scene` as a prefab, in the sense given by Unity: one-way irreversible spawning of a group of entities and components.

## What solution would you like?

No `SceneBundle`, no `SceneInstance`, no `SceenSpawner`. Just a way to spawn the content of a scene anywhere (including at the `World` root, without a common parent entity, so ability to spawn multiple top-level entities as a flat list) and forget forever that this ever was a `Scene`.

## What alternative(s) have you considered?

- `SceneBundle`. This has all the mentioned extra "features" which are just getting in the way most of the time for typical "use a prefab to spawn multiple things" usage. This is also missing some features like spawning without a single comment parent entity.
- Spawning function. This only works if you hardcode the collection of entities and components, and is hard to work with (spawning via code is tedious).

## Additional context

Possibly related to #3877, #10907

Start by reviewing the related issues #3877 and #10907, then compare the requested behavior with the existing SceneBundle and SceneInstance concepts mentioned here. Clarify how scene content should be spawned at the World root or as a flat list of top-level entities, and consider the request complete when a documented approach supports one-way spawning without retaining scene-instance behavior.

Written by the indexing model from the issue text.