Last summer, Posh was just another stray cat, living out his days at Quakertown's "Last Chance Ranch." Now, the chunky feline with deep green eyes has a home in a Delaware Valley College dorm. Posh playfully crawled last week across an organic chemistry notebook left open on the dorm-room desk. "No one's real favorite, except for, I guess, his," observed Russell Desmond, a West Chester sophomore and aspiring veterinarian who with his roommate, Nate Borger, shares custody of the cat.