Sphere constructor
Creates a sphere with the specified radius and optional center.
radius must be positive.
center defaults to (0, 0, 0).
Implementation
Sphere(this.radius, {Point? center})
: center = center ?? Point(0, 0, 0),
super('Sphere') {
if (radius <= 0) {
throw ArgumentError('Radius must be positive, got: $radius');
}
}