One unspoken premise of improvisation is that serendipity can yield musical moments beyond the scope of premeditation. Trumpeter Christian Scott aTunde Adjuah’s third live album, Axiom, features many such moments — grooves shift weight to expose counter-rhythms, and scribbled climaxes heave irrepressibly out of tranquil expanses. But in this case, the existence of the album is serendipitous in and...