On the first day of the Cardinals first full-squad workouts, a dedicated smattering of Cardinals loyalists gathered behind chain-link fences in small clusters way off in the distance. They all had come to get a glimpse of “The Phenom.” But his spring-training debut on practice field No. 1 not only was shortened by cool and steady rains, it was driven under cover to an isolated, tin-roofed practice cage that wards off interlopers with a big foreboding sign: “Players and Coaches Only Beyond This Point.” In so many ways, this was the perfect way for 20-year-old Oscar Taveras’ first major-league spring training to begin; any decent sighting of him limited by a strict “need-to-know-only” basis that only heightens the delicious intrigue. Few spring training rituals are more entertaining than the annual fascination with the unproven but potentially dazzling whiz kids who have yet to play a single inning in the big leagues. Everybody loves ‘‘The Phenom,’’ and the legend of the wiry outfielder who has been tearing up the minor leagues for four dazzling summers has reached breathless levels among Cardinals Nation. He’s universally considered one of the three top hitting prospects in the minor leagues, as intriguing a talent around this organization since a kid named Albert Pujols was knocking the cover off the ball in Jupiter more than a decade ago. “I saw him twice as a (minor-league) instructor,” said manager Mike Matheny. “And I tell you, he stood out. Impressive.”