In-memory state shared by all gRPC handlers (new + legacy). All ops go
through one of the typed async methods below. Arc<AtlasRegistry> is
cheap to clone; the interior RwLock serialises mutations.
Start the Atlas gRPC server on listen using the given registry.
Spawns a background heartbeat-eviction task and exposes BOTH the new
Atlas service and the deprecated RobonixRuntime shim on the same
gRPC port.