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>

src/lib.rs

@@ -6,6 +6,7 @@ pub mod config;
 pub mod ctx;
 pub mod forward;
 pub mod header;
+pub mod lan;
 pub mod override_store;
 pub mod packet;
 pub mod proxy;