Builds a MeasureContext from an existing RenderContext — used by composite blocks (stack/section) that need to measure each child "in the middle of" rendering, to do per-child ensureSpace the same way the engine's render loop does for top-level blocks (keeps a child that overflows the remaining space from spilling off the page without breaking)
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.