Move CSS animation values into a mutable overlay on computed properties
and make base computed style data immutable after construction. Base
style mutation now goes through a builder that is consumed on publish,
so installed styles no longer expose mutation APIs.
Build new base style data for inherited style updates instead of cloning
and mutating installed computed properties. Element-specific computed
style adjustments now run before publication, while animation and
transition updates continue to mutate only the animated overlay.
