Spawnable Observers

## Goal Description

Observers watching a newly spawned entity should be as easy to create as the inclusion of components and relations. It should be possible to easily spawn observers in code and as part of scenes, without any terrible performance footguns or surprising quirks.

Interacts with #20158. Previously: #17607, #14204, #22561.

## Goal Status

This is a potential [Bevy Project Goal](https://bevy.org/learn/contribute/project-information/project-goals/). Its state on the [Project Goals Board](https://github.com/orgs/bevyengine/projects/23/views/1) determines if it is Proposed, Postponed, Blocked (Approved), Inactive (Approved), Active (Approved), Completed, or Declined.

- **Staffing SMEs**: Not yet staffed
- **Working Group Link**: Working Group not yet formed
- **Design Documents**: Design documents not yet written

Start by reading the related issues #20158, #17607, #14204, and #22561, then review the project-goal status. No implementation files, tests, entry points, or design documents are identified yet; this issue is done only when a defined design and implementation path exists for spawnable observers in code and scenes.

Written by the indexing model from the issue text.