P2P bootstrap via master discovery topic + seed nodes

cruiser Phase 30 lands: a master gossip topic that all nodes join on first boot, plus a small set of seed nodes pinned in the binary. Once you've heard one peer through the master topic, you can drop into per-app sub-topics and never speak to the master again. Trade-off: the master topic centralises discovery (everyone hits it once), but at least it's not a TLS-terminating tracker — just an…