reflowTargetSizes

Natural sizes captured for every reflow node this commit (changed and unchanged), keyed by LithoNode.transitionGlobalKey; the commit itself is pinned to the previous/in-flight size. StyleTransitionsExtension reconciles each against the live store target at mount, so a background walk that read a stale target cannot make the wrong retarget decision. Null when the tree declares no reflow node, or when every reflow node was driving its own animation frame.