minip2p v0.1.1

The pubsub facade now selects gossipsub by default. This is a pre-1.0 behavior change: EndpointBuilder::pubsub_config accepts either GossipsubConfig or FloodsubConfig through PubsubConfig; applications that require the old engine must pass FloodsubConfig::default() explicitly. Each engine advertises only its own protocol IDs, so mixed gossipsub/floodsub endpoints intentionally do not exchange pubsub traffic.