Most invalidation sets carry at most one selector feature. Storing
every set in a HashTable pays allocation and hashing costs while
constructing style invalidation data, even in the common case.
Store the first property inline and upgrade to HashTable only when a
second distinct property is inserted. Keep hash and equality checks
direct over the backing so those hot paths do not route through the
public callback iterator.
Compare invalidation sets, rules, and plans structurally so repeated
descendant and sibling invalidation entries can be merged even when
they were built as separate payload objects.
Also deduplicate pending and active descendant invalidations in the
style invalidator so equivalent rules are not re-applied as the DOM
walk descends. This reduces :has() invalidation fanout while keeping
behavior the same.
Implements idea described in
https://docs.google.com/document/d/1vEW86DaeVs4uQzNFI5R-_xS9TcS1Cs_EUsHRSgCHGu8
Invalidation sets are used to reduce the number of elements marked for
style recalculation by collecting metadata from style rules about the
dependencies between properties that could affect an element’s style.
Currently, this optimization is only applied to style invalidation
triggered by class list mutations on an element.
