Six Hong Kong developers have agreed to pay HK$12.6 billion (US$1.6 billion) for the third-largest plot of residential land on the runway of the former Kai Tak airport, forking out a record price for the prime oceanfront real estate as a property bull market resumes in the world’s costliest city.The government tender for Kai Tak Area 4C Site 2 was won by the Marble Edge Investment consortium, comprising China Overseas Land & Investment, Chinachem Group, Empire Development, Henderson Land…