LocalFlexBoxScope

Carries the innermost FlexBox's Compose FlexBoxScope down the tree, or null when no ancestor FlexBox has provided one — read by Modifier.size() to resolve a child's Flex sizing behavior (grow/shrink/basis/align-self/order). Provided by FlexBoxTileRenderer; unlike LocalColumnScope/LocalRowScope/LocalLazyItemScope, no other container renderer resets this back to null, so it keeps propagating through any descendant subtree until another FlexBox provides a fresh value.