amp-script: binding can cause real DOM and virtual DOM to get out of sync

I'm not sure there's a great solution to this one. I'm just reporting it as an issue in case someone sees a solution... and otherwise I'm just documenting the behavior.

If another process modifies an ``'s children, that change won't propagate to the virtual DOM - and the user's JavaScript will see stale data.

Here's one way in which this can happen:

```html

Will I change?

Change this and amp-script won't know
```

Reproduce the example using amp-script child bindings and AMP.setState, then trace how the real DOM and virtual DOM are updated. No source files or tests are named; done would require an agreed solution or documented behavior that keeps user JavaScript from observing stale data.

Written by the indexing model from the issue text.