Function add

Adds the character to the physics world by creating an inner rigid body.

The inner body serves multiple purposes:

1. Visibility: Regular raycasts can hit the character
2. Contact Callbacks: Physics Listener receives callbacks
3. CCD: Fast objects won't pass through character
4. Sensors: Character can trigger sensor volumes

Note: Inner body collision does NOT affect character movement. The character controller's own collision detection is separate.

Requires innerRigidBody to be set on the character.