Respond to asset changes using observers.

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

It would be quite ergonomic to be able to respond to when assets get loaded, changed, etc, using observers. We currently have `AssetEvent`, but the way currently things stand in Bevy, old-style events (using `EventReader`/`EventWriter`) and observer triggers are actually different workflows that are incompatible with one another, even though they both (confusingly) use the same trait (`Event`).

## What solution would you like?

Bevy to "trigger" observer-style events when assets get loaded/changed/etc.

## What alternative(s) have you considered?

Just using `EventReader`

## Additional context

If Bevy ever moves to "Assets as Entities" (as was attempted during the AssetsV2 rework, but ultimately dropped in favor of using `Assets` resources as before), this would become even more ergonomic, as the observer trigger could "target" the entity that represents the affected asset.

Start by reading the existing AssetEvent workflow alongside EventReader/EventWriter and observer triggers. The issue names no files or tests, so first identify the relevant asset and observer entry points, then clarify the event semantics and validation needed before implementation.

Written by the indexing model from the issue text.