Boo doesn’t know it, but he has a big role to play in his human companion’s weight-loss plan. Roseburg’s Tawnya Watson scrapes food off her plate for the 4-year-old Labrador-border collie cross when she’s cutting back on portion sizes. Probably better for him, Boo joins Watson and a second family dog, Fe, when Watson hits the pavement headed for Fir Grove Park or Stewart Park. Her goal is to stretch the walks from three to six times a week. That’s not easy for a mother of three teens. “If left to my own devices, I tend to put me at …