Add readiness probe to devenv

devenv.nix

@@ -74,6 +74,12 @@ in
         --mountpoint /tmp/rust-fuse \
         --database "postgresql://fujin@localhost/musicfs?host=$PGHOST"
     '';
+    # Probes the standard gRPC Health Checking Protocol exposed on the
+    # musicfs-client status server (default --listen 127.0.0.1:50052).
+    # SERVING implies the FUSE mount succeeded and, for NetworkOrigin, the
+    # upstream musicfs-server is reachable (health.rs toggles not_serving on
+    # upstream loss). Inherited by profiles.* since they only override `exec`.
+    ready.exec = "grpcurl -plaintext -max-time 2 127.0.0.1:50052 grpc.health.v1.Health/Check | grep -q SERVING";
   };
 
   profiles.local.module = {