Joints and CharacterController

CharacterController on PhysX's capsule controller, as Unity's:
- Move with collide-and-slide, stepOffset, slopeLimit, skinWidth and minMoveDistance.
- CollisionFlags (not a [Flags] enum, as in Unity), isGrounded and velocity.
- SimpleMove: gravity accumulates, and horizontal speed applies only while grounded (recorded).
- OnControllerColliderHit carries PhysX's per-pass direction and length, through a hand-built vtable for PxUserControllerHitReport.
- The controller is a collider for triggers and queries. Move doesn't sync transforms; script teleports are overwritten.
- character_controller: 220 checks.
3D joints (PhysX):
- FixedJoint, SpringJoint (distance joint) and HingeJoint (D6: twist motor, limits, spring).
- ConfigurableJoint (D6 motions, linear/twist/swing limits, drives) and CharacterJoint.
- Breaking, with OnJointBreak, and currentForce/currentTorque.
- Unity's conventions: actor 0 is the connected body; angles from the start pose; auto connected anchors computed when configured.
- joints_basic: 197 checks.
2D joints (Box2D): HingeJoint2D, DistanceJoint2D (rod and rope), SpringJoint2D, FixedJoint2D, SliderJoint2D, WheelJoint2D, TargetJoint2D.
- Breaking by reaction force, as Unity does, with OnJointBreak2D and breakAction.
- Unity's per-type auto-anchor defaults, including the wheel's connected-body origin.
- Box2D 2.3's prismatic limit slop.
- joints2d_basic: 229 checks.
Scenes: Unity's settings structs (JointSpring, JointLimits, JointMotor2D...) as JSON objects; Infinity and NaN floats as strings.
Sample: joints_playground.json, played by an MCP e2e test: the controller pushes a crate, crosses the hinged bridge and opens the door.
Next: AudioSource, SceneManager and PlayerPrefs (done in slice 5).