`AppExit` should be an observer event rather than a `BufferedEvent`

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

Multiple `AppExit` events in a buffer doesn't make sense, and would be better modeled as an observer event. This would be more performant for plugins to react to app exits, rather than having to schedule a system that runs every frame.

## What solution would you like?

Swap `AppExit` to an observer event.

## What alternative(s) have you considered?

Weird status quo.

Start by locating the AppExit definition and its BufferedEvent usage, then read the surrounding event and observer APIs to understand the existing pattern. Done means AppExit is modeled as an observer event, multiple buffered instances are no longer possible, and the relevant event behavior remains covered by passing tests.

Written by the indexing model from the issue text.