LibWeb: Invalidate scoped styles when boundaries change

Scoped rules depend on their `@scope` start and end selectors, not just
the selectors for the declarations inside the rule. Register those
boundary selectors in the style invalidation data and selector insights
so mutations that create or remove scope roots or limits invalidate
descendant styles that may now match differently.

Scope boundary matching also needs to record selector involvement
against the element being styled. That keeps :has(), sibling, and
structural selector invalidation from treating the boundary candidate
as the only affected subject.

Run the stored :has() invalidation plans for affected anchors as well
as marking subject anchors dirty. This lets scope-boundary :has()
changes invalidate scoped descendants instead of leaving stale styles.