DynamicChildrenNode
Privileged retained capability that materializes only the direct children needed by the next measure pass.
Implementations are owner-thread confined and must return immutable descriptions with stable keys when identity survives range movement. The runtime validates and reconciles the returned sibling set before measurement, then owns the descriptions exactly like ordinary declarative children. State changes that alter the returned set must invalidate DirtyPhase.Measure. The callback must not mutate session state or recursively operate on the owning tree.