Hong Kong's stock exchange reported its highest quarterly profit in nearly four years after China's stimulus measures boosted trading and listing volume. BEIJING — Mainland Chinese investors are piling into the Hong Kong stock market at record volumes as its tech-heavy Hang Seng Index trades around three-year highs. Net mainland Chinese purchases of Hong Kong stocks hit a record 29.62 billion Hong Kong dollars ($3.81 billion) on Monday, according to the Wind Information database. That was the most since the Hong Kong stock market launched its "connect" program with the mainland, allowing local investors easier access to a select number…