The reckless, splendid circus of New York's royal bohemia in the 1960s and '70s — rock idols, cowboy poets, Warhol Superstars — surrounds Smith in her heady recounting of a halcyon era. But the heart of Just Kids, a captivating memoir, is the lifelong love affair (first romantic, later creative and platonic) between Smith and photographer Robert Mapplethorpe, before she became music's punk poet laureate and he one of the art world's most provocative and controversial figures. In her inimitable, lyrical style, Patti Smith recalls the pair's coming together as young, monumentally broke dreamers: ''just kids.'' What follows is both a poignant requiem (Mapplethorpe died of AIDS at age 43) and a radiant celebration of life. A