Curtis Martin was a solitary figure standing on the sideline yesterday, clad in a gray T-shirt and gray sweat pants. His face was covered with the scowl of a warrior longing to go to battle with his teammates. The temperature, in the mid-90s, only intensified the image.Martin, placed on the physically unable-to-perform list on Thursday as his right knee has been slow to heal following arthroscopic surgery in December, is experiencing a different kind of pain these days.