feat(nat): birthday attack module + HardNatBirthdayStart signal (#86, #87)

Birthday attack for random symmetric NATs:
- birthday.rs: open_acceptor_ports() opens N sockets, STUN-probes
  each to learn external ports. generate_dialer_targets() builds
  hit list (known ports first, then random fill). spray_dialer()
  sprays QUIC connects with rate limiting, first success wins.
- Default: 32 acceptor ports, 128 dialer probes, 20ms interval

Signal coordination:
- HardNatBirthdayStart { acceptor_ports, external_ip } sent by
  Acceptor when peer's HardNatProbe shows random/sequential NAT
- Relay forwards it like other call signals
- Desktop recv loop handles and logs it

Hybrid waterfall integration:
- On receiving HardNatProbe with non-cone allocation, Acceptor
  auto-opens birthday ports and sends BirthdayStart
- Sockets kept alive 10s for NAT mapping persistence
- Dialer spray integration into race() pending (needs transport
  hot-swap for background upgrade)

6 new tests, 599 total, 0 regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/src/main.rs

@@ -1443,8 +1443,9 @@ async fn main() -> anyhow::Result<()> {
                                     }
                                 }
 
-                                // Hard NAT: forward HardNatProbe to call peer
-                                // (same forwarding pattern as CandidateUpdate).
+                                // Hard NAT: forward HardNatProbe + HardNatBirthdayStart
+                                // to call peer (same pattern as CandidateUpdate).
+                                SignalMessage::HardNatBirthdayStart { ref call_id, .. } |
                                 SignalMessage::HardNatProbe { ref call_id, .. } => {
                                     let (peer_fp, peer_relay_fp) = {
                                         let reg = call_registry.lock().await;