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.
