By Wally Pyrah NO PRIZES for guessing who the headline act will be when racing gets underway with a nine-race programme at Happy Valley today. Champion Jockey Zac Purton reached new heights at Sha Tin last Sunday, riding his first ever seven-timer and sending home his many thousands of supporters with bulging pockets full of dollars. The Zac-Man can do no wrong at present, with 21 victories after just nine meetings and a staggering 13 wins from his last 45 mounts. With another full book of rides at the city track where he is the dominant force, you can guarantee there will be long queues at th…