As Hainan pushes ahead with its free-trade port, the tropical island is emerging as a potential rival to Hong Kong’s long-standing role as a regional shopping and low-tax hub – even as both sides signal interest in closer cooperation.Since launching a separate customs regime three months ago, Hainan – now a gateway to the Chinese market – has positioned itself as a “super partner” to Hong Kong across industry chains, finance, tourism and other sectors, according to officials.“Hong Kong serves...