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-proto/src/packet.rs

@@ -967,6 +967,19 @@ pub enum SignalMessage {
         external_ip: String,
     },
 
+    /// Birthday attack coordination — Acceptor tells Dialer which
+    /// ports it has open. The Dialer then sprays QUIC connects to
+    /// these ports (and optionally random ports) on the Acceptor's IP.
+    HardNatBirthdayStart {
+        call_id: String,
+        /// Number of sockets the Acceptor opened.
+        acceptor_port_count: u16,
+        /// External ports discovered via STUN (the "hit list").
+        acceptor_ports: Vec<u16>,
+        /// Acceptor's external IP.
+        external_ip: String,
+    },
+
     // ── Phase 4: cross-relay direct-call signaling ────────────────────
 
     /// Phase 4: relay-to-relay envelope for forwarding direct-call