A season is always the sum of its parts. The pieces of Early Spring are few and subtle, but Middle Spring, reaching its zenith this week, leaves little to the imagination. The meager inventories of change that characterized equinox quickly fill with new details. Trees leaf and flowers bloom, unmistakable, their numbers catching the eye of almost everyone. All the habitats, from rich talus slopes to alleyways, reveal the prints of change. The first leaf is out on the lizard's tail. The first wild