Scene Graph unit tests move to packages/scene-graph/tests with a package-local assert helper and test type-checking; five Core- and fig-owned tests move to their owners' engine homes. check:test-homes rejects any new test under tests/engine against a reviewed baseline so the migration debt only shrinks.
- Move scene graph behavior tests into focused files
- Share basic scene graph helper builders from basic/helpers.ts
- Remove scene graph basic from max-lines exceptions
