CAN the 100th episode of a money-spinning serial ever be a routine affair? No way. So the producers of Aahat, which has of late emerged as the principal source of Sony Entertainment Television's growing clout in the cable and satellite television market, have pulled out all the stops. They've assembled an impressive cast of actors headed by Om Puri, thrown in the best special effects and camerawork that TV can afford and, most important, conjured up a storyline that vociferously pleads for a willing suspension of disbelief but, predictably, doesn't always get it.