“The mood is, by turns, anxious and dark, even sinister, but also, at times, expectant, guardedly hopeful. Everyone is on edge. The show presents a nation, and the sensibilities of its artists, in a period of transition, with violence cresting, identities in flux, and some brave souls hatching plans. A sea change is coming, though it is unclear if its effect will be disastrous, momentous, or something more complicated. Call it the biennial on the brink.”