This change fixes issues with screen order comparison logic when rects are encompassed within each other. This was causing issues when trying to select text that includes inline `WidgetSpan`s inside of a `SelectionArea`.

* Adds `boundingBoxes` to `Selectable` for a more precise hit testing region.

Fixes #132821
Fixes updating selection edge by word boundary when widget spans are involved.
Fixes crash when sending select word selection event to an unselectable element.