Color contrast may need to work across frame boundaries

The HTML element of a page loaded in an iframe has opacity of 0. Because of this, the text in an iframe can be displayed on top of a background color that is in its parent frame:

I.e; Axe should not pass this:

```html

Ideally axe-core should have an after method for its color contrast rules that can compute contrast of elements across frame boundaries. That's quite a big restructure for something that in practice has never been reported, so a simpler thing we could do is set incomplete for nodes inside a frame when there is no background.

Start by reproducing the iframe example and trace axe-core’s color contrast rules and frame-boundary handling. Compare the proposed cross-frame contrast calculation with marking nodes incomplete when no background is available, then confirm the expected behavior and cover it with a regression test.

Written by the indexing model from the issue text.