crossing(particle) {
    const d = particle.p.distanceTo(this.center);

    if (this.crossType === "dead") {
      if (d - particle.radius > this.radius) particle.dead = true;
    } else if (this.crossType === "bound") {
      if (d + particle.radius >= this.radius) this.getSymmetric(particle);
    } else if (this.crossType === "cross") {
      if (this.alert) {
        console.error("Sorry, CircleZone does not support cross method!");
        this.alert = false;
      }
    }
  }
Handles particle crossing behavior.
@param {Object} particle - The particle to check for crossing.