A @keyframes rule scoped to a shadow root was not reliably reached
from an animated slotted light-DOM element: the keyframes lookup
walked the element's own root first, then fell back to the document,
but slotted elements can pick up animation-name from a ::slotted(...)
rule that lives in an ancestor shadow root rather than in the
element's own tree.
Track the shadow-root scope that supplied each winning cascaded
declaration, and use that scope to resolve the matching @keyframes
when processing animation definitions. A shared constructable
stylesheet can be adopted into several scopes at once, so the
declaration object alone is too weak as a key; the per-entry
shadow-root pointer disambiguates which adoption actually contributed.
Also refresh running CSS animations' keyframe sets when style is
recomputed. Previously only the first animation creation path set a
keyframe set, so an existing animation never picked up newly inserted
@keyframes rules.
