Thanks! @theFlu (I never thought I'd thank the flu, but hey. Blunders never cease)
I poked around in Data/Config/gamestages.xml and found an object named "feralHordeStageGS4086" but none numbered higher. Looks like I'm far away from the max game stage. But I still don't understand why my game stage has quit increasing. Perhaps it stops once you reach max character level (300)? Though without demolishers, horde nights are a way less exciting. 
Going back to to logs, I see that horde night began with spawns from feralHordeStageGS1269, then feralHordeStageGS1282, and ended on feralHordeStage1295. All rather odd as the player table (the i key) lists gamestage as 1296.