SushiEngine::Simulation::PhysicsJointParameters
connected_body
EntityId connected_body = NULL_ENTITYDescription
The entity this one is attached to; NULL_ENTITY attaches nothing.
A joint naming no partner, or naming an entity with no rigid body, is authoring in progress rather than an error — an author picks the kind before they pick the partner. It simply does not become a live joint, and the inspector says why rather than the scene silently omitting it.

