Concinnity physics: the engine's rigid-body simulation and the vocabulary
it is driven through, in its own crate so it names no engine domain type.
It provides the shapes, body parameters, joints, layer masks, and step
results that cross the boundary between a caller and the [Simulation],
and the [PhysicsBudget] a world reserves its bodies against.
Everything crossing that boundary is plain data in the engine's [f32; 3]
and Euler-degree representation, addressed by an opaque [BodyHandle].
That is what lets the simulation hold whatever math types it likes without
those types reaching a caller.