By Jude Summerfield Manchester United have confirmed they have signed highly-rated young forward Chido Obi-Martin after he left Premier League rivals Arsenal. The 16-year-old’s schoolboy contract with the Gunners expired over the summer and it was reported United were exploring a potential deal for the record-breaking striker. Obi-Martin, who scored 32 goals in just 18 games for Arsenal’s Under-18s last season, was also said to have been won over by the Red Devils following contact with legendary striker Ruud van Nistelrooy, who is on Erik ten Hag’s coaching staff at Old Trafford. United have …