The pieces a composite custom block needs and could not reach before: render() only ever
receives a RenderContext, so without deriveMeasureContext there was no way to measure children
at all, and measureBlocksHeight/buildConfinedContext are what the built-in containers
(stack/row/box) use to size and confine theirs.
Total natural height of a list of blocks — the "composite measure" every composite shares
(stack/section/box/row column/bottomAnchor, plus the engine measuring a band's blocks).
Sums each block's own measureHeight against the same context.
The pieces a composite custom block needs and could not reach before: render() only ever receives a RenderContext, so without deriveMeasureContext there was no way to measure children at all, and measureBlocksHeight/buildConfinedContext are what the built-in containers (stack/row/box) use to size and confine theirs.