MOSTAR-A DEVASTATED NO-MAN'Sland partitions the city of Mostar into a Croat-governed west zone and a Muslim-governed east zone, and the European Union has the daunting challenge of reuniting and reconstructing the two halves. With its two-year mandate scheduled to expire in July 1996, the EU is working with increasing urgency. The EU's efforts to reconstruct a multi-ethnic society in Mostar have been the cornerstone of European and American hopes for a successful Bosnian-Croat Federation.