reapOnStop

The lane's one terminal boundary, signaled from outside the pump: MinecraftServer#stopServer (Paper: onDisable). Past it the heartbeat never fires again, so every queued eval would hang the request blocked on it — including one still compiling, which will offerOrReap itself into the queue a moment later. Advance the epoch before draining — see offerOrReap for why that pairing strands nothing. Server thread only, and the sole writer of currentEpoch.