MP Materials shares surged on Wednesday after the company announced a joint venture with Saudi Arabia’s Ma’aden to build a rare-earths processing and magnet manufacturing hub in the kingdom, a move that positions the U.S. producer inside one of the world’s fastest-growing critical-minerals investment programs. The partnership will give MP a new downstream base in the Gulf as Saudi Arabia accelerates its bid to become a major player in the global rare earth supply chain.The companies will establish a refinery and separation plant…