Plus with those same resources you could probably build a solar-system-sized passive interferometric radar swarm, and get decent warning time on any relativisti

Plus with those same resources you could probably build a solar-system-sized passive interferometric radar swarm, and get decent warning time on any relativistic launches. 0.99c of blueshift coming from a specific star's direction? Weird. Shoot a couple relatively slow BBs at it, and the projectile will vaporize itself around the Kuiper Belt. Poor guys. They've now announced to the universe that they're an aggressive threat to all life around them…