Keep StyleScope responsible for storing pending :has() mutation state,
but move the invalidation walk and scheduling helpers into
CSS::Invalidation::HasMutationInvalidator. This keeps the style scope
from owning the :has() invalidation algorithm directly and gives later
changes a narrower place to optimize.
Document.cpp still flushed pending :has() invalidation by walking the
document and shadow-root style scopes directly. Move that CSS-specific
flush into CSS::Invalidation::HasMutationInvalidator.
Document continues to own the flag that says a :has() flush is needed.
The helper now owns the style-scope work needed to invalidate elements
affected by pending :has() mutations.
Element exposed a small method that encoded how :has()-affected elements
are marked dirty. Move that policy into CSS::Invalidation alongside the
rest of the :has() mutation invalidation helpers.
This keeps Element focused on DOM state while preserving the existing
subject and non-subject :has() invalidation behavior.
Node.cpp still contained the policy for deciding when a DOM mutation
should schedule pending :has() invalidation work. Move that into
CSS::Invalidation::HasMutationInvalidator, next to the mutation feature
collector it depends on.
This keeps DOM mutation code focused on reporting that a mutation
happened, while CSS invalidation code owns the selector-specific checks
for :has() metadata and sibling-combinator sensitivity.
