Force Out on Trapped Ball

 
The batter grounds to the shortstop, who bounces a throw over to first base. The first baseman "traps" the ball against his body (his rib-cage) with his glove-side forearm. The first baseman holds the ball firmly and securely in this manner against his body while the batter-runner crosses first base. After the batter-runner crosses first base, the first baseman pulls the ball out with his non-gloved hand and shows it to the umpire, proving that he had possession of the ball.
 
Is the batter-runner safe or out?
 
Call
The fielder must have firm/secure control of the ball in either his hand or glove. The fielder did not meet this criteria until after the batter-runner crossed the base, so the runner is safe. Had the first baseman pulled the ball away from his body and held it in his hand before the runner crossed the base, the runner would have been out.