HE3D physics. HE3D 物理模块。
A thin [PhysicsWorld] adapter over the standalone [gp3d] engine, mapping
render objects by stable [ObjectId] onto gp3d's generational body handles.
一个建立在独立 [gp3d] 引擎之上的薄 [PhysicsWorld] 适配层，通过稳定的
[ObjectId] 将渲染对象映射到 gp3d 的代际刚体句柄上。
Bodies are owned by gp3d; after each [PhysicsWorld::step] the caller synchronizes
transforms back into the [Scene].
刚体由 gp3d 持有；每次 [PhysicsWorld::step] 之后，调用方将变换同步回 [Scene]。