Xiaomi's new foldable appears to be off to a very strong start. The Xiaomi 18 Fold sold around 36,000 units on its first day, despite carrying a price that puts it firmly in ultra-premium territory.Even more impressive is how that compares with Xiaomi's previous foldable. The company says sales jumped 310% compared with the previous generation in the same price segment.36,000 phones sold on the first dayThe Xiaomi 18 Fold was announced on September 7 before going on sale in China on September 10. According to industry insider RD Observation, Xiaomi sold around 36,000 units during that first day. That…