add LAN service discovery via UDP multicast

Numa instances on the same network auto-discover each other's .numa
services. No config, no cloud — just multicast on 239.255.70.78:5390.

- PeerStore with lazy expiry (90s timeout, 30s broadcast interval)
- DNS resolves remote .numa services to peer's LAN IP (not localhost)
- Proxy forwards to peer IP for remote services
- Graceful degradation if multicast bind fails

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -22,6 +22,7 @@ hyper = { version = "1", features = ["client", "http1", "server"] }
 hyper-util = { version = "0.1", features = ["client-legacy", "http1", "tokio"] }
 http-body-util = "0.1"
 futures = "0.3"
+socket2 = "0.5"
 rcgen = { version = "0.13", features = ["pem", "x509-parser"] }
 time = "0.3"
 rustls = "0.23"