MeasureScope

Direct-child measurement scope for one parent measurement pass.

The scope is owned by the MeasureNode.measure callback and is valid only until that callback returns. Every access must use the tree's owner thread. Access after the callback or from another thread fails. Each direct child can be measured at most once through this scope. An index outside the direct-child range or a second measurement of the same child throws. The tree is poisoned once pipeline work has started only when that exception escapes the owning measure callback.