PASS: no backward-structural rule: append does not invalidate previous siblings | previousSiblingInvalidationWalkVisits=0
PASS: forward-only rule (:first-child): append does not invalidate previous siblings | previousSiblingInvalidationWalkVisits=0
PASS: backward-structural rule on unrelated subtree: append does not invalidate previous siblings | previousSiblingInvalidationWalkVisits=0
PASS: last-child rule on fixture: append targets previous last child without walking | previousSiblingInvalidationWalkVisits=0
PASS: nth-last-child rule on fixture: append DOES walk previous siblings | previousSiblingInvalidationWalkVisits=20
