feat(nat): smart candidate filtering + acceptor NAT tickle + 4s timeout

Major P2P improvements for cross-network calls:

Smart candidate filtering (smart_dial_order):
- Strip LAN candidates when peer's public IP differs from ours
  (172.16.x.x is unreachable from a different network)
- Strip all IPv6 candidates (Phase 7 disabled, wastes dial slots)
- Only keep mapped + reflexive for cross-network calls
- LAN candidates preserved when both peers share the same public IP

Acceptor NAT tickle:
- A-role sends a 1-byte UDP packet to each peer candidate BEFORE
  accepting. This opens the NAT pinhole for return traffic from
  the Dialer's IP — critical for address-restricted NATs that only
  allow inbound from IPs they've seen outbound traffic to.
- Uses SO_REUSEADDR on the same port as the quinn endpoint.

Direct timeout increased from 2s to 4s:
- Cross-network QUIC handshakes through CGNAT can take 2-3s
- 2s was too aggressive for 5G/LTE networks

Diagnostic fix:
- Record "timeout:4s" for candidates still in-flight when the
  timeout fires (previously these had no diagnostic entry)

5 new tests for smart_dial_order edge cases.
593 tests pass, 0 regressions.

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

desktop/src-tauri/src/lib.rs

@@ -476,12 +476,15 @@ async fn connect(
 
                 let room_sni = room.clone();
                 let call_sni = format!("call-{room}");
+                let own_reflex_parsed: Option<std::net::SocketAddr> =
+                    own_reflex_addr.as_deref().and_then(|s| s.parse().ok());
                 match wzp_client::dual_path::race(
                     r,
                     candidates,
                     relay_sockaddr,
                     room_sni,
                     call_sni,
+                    own_reflex_parsed,
                     signal_endpoint_for_race.clone(),
                     ipv6_endpoint_for_race.clone(),
                 )