s&box: Physics joints — hinge, spring, rope, fixed constraints with PhysicsJoint factory methods

In s&box, physics joints (hinges, springs, ropes, fixed constraints) are created via PhysicsJoint static factory methods on the PhysicsBody level, not as Components. Developers look for a HingeJoint component like Unity but the API is lower-level. There are also scene-level Joint components for editor-configured joints.