BalanceConfig
data class BalanceConfig(val makeTree: (children: List<Any>, positions: List<Int>, length: Int) -> Tree? = null)
Configuration for Tree.balance.
makeTree builds the inner grouping nodes. It defaults to a factory producing anonymous (NodeType.none) trees; the tree it returns is used as-is, so a caller can attach props of its own. It is never used for the top node, which always keeps the balanced tree's own type and props.