Flutter render objects

This skill is the renderobjects.com guides, API pages and examples by Matt Carroll (https://renderobjects.com/), with every API claim checked against api.flutter.dev and the Flutter framework source. The site states no Flutter version, and some of its code does not compile or contradicts the framework; wherever that happens the files give the official form first and then one line saying what renderobjects.com has. What decides most outcomes: the element tree owns the children and the render tree only reacts, every property setter marks exactly the right thing dirty, and whatever a render object reports without laying out (dry size, baselines, intrinsics) matches what its real layout produces. App-level frame budget and the advice to avoid intrinsic passes are in flutter PERFORMANCE.md; tap targets, the Semantics widget and screen-reader checks are in flutter ACCESSIBILITY.md; everything else about building the app is flutter SKILL.md.

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