PORT ST. LUCIE, Fla. — The Mets have made a decision about struggling DH Darin Ruf, designating him for assignment. The 36-year-old right-handed hitter will not be included on the club’s Opening Day roster. Instead, the Mets will select outfielder Tim Locastro, a speedy base-stealing specialist. The Mets can take advantage of the new rules that encourage more action on the basepaths with Locastro, a speedy base-stealing specialist. Ruf was acquired at the trade deadline last season from the San Francisco Giants.