The Kiwi codec types only symbolID, so every test reading symbolOverrides
or uniformScaleFactor repeated the same cast against a type the package
exported without the readers that go with it. symbolDataOf and
symbolOverridesOf are now exported and five call sites use them.
tests/engine/library exercises @/app/libraries, so it belongs under
tests/app by the placement rules; the shard list and the engine baseline
follow it.
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