fix(net): bind all endpoints to [::]:0 for dual-stack IPv4+IPv6

Every QUIC endpoint was bound to 0.0.0.0:0 (IPv4-only). This
silently killed ALL IPv6 host candidates: the Dialer couldn't
send packets to [2a0d:...] addresses (wrong address family on
the socket), and the Acceptor couldn't receive incoming IPv6
QUIC handshakes. The IPv6 candidates were gathered and advertised
in DirectCallOffer/Answer but were completely non-functional.

On same-LAN with dual-stack (which both test phones have), this
meant:
- JoinSet fanned out 3+ candidates (2× IPv6 + 1× IPv4)
- IPv6 dials failed silently or timed out
- IPv4 dial worked but competed with failed IPv6 for JoinSet
  attention
- Sometimes the JoinSet returned an IPv6 failure before the
  IPv4 success, causing unnecessary fallback to relay

Fix: bind to [::]:0 (IPv6 any) instead of 0.0.0.0:0. On
dual-stack systems (Linux/Android default), [::]:0 creates a
socket that handles BOTH:
- IPv6 natively (global unicast, ULA)
- IPv4 via v4-mapped addresses (::ffff:172.16.81.x)

One socket, both protocols. All 7 bind sites updated:
- register_signal (signal endpoint)
- do_register_signal
- ping_relay
- probe_reflect_addr (fresh endpoint fallback)
- dual_path::race (A-role fresh, D-role fresh, relay fresh)

With this fix, same-LAN P2P should prefer the IPv6 path (no
NAT, direct routing, lower latency) and fall through to IPv4
if IPv6 fails — relay is the last resort after ALL candidates
are exhausted.

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

desktop/src-tauri/src/lib.rs

@@ -194,7 +194,7 @@ fn get_call_debug_logs() -> bool {
 async fn ping_relay(relay: String) -> Result<PingResult, String> {
     let addr: std::net::SocketAddr = relay.parse().map_err(|e| format!("bad address: {e}"))?;
     let _ = rustls::crypto::ring::default_provider().install_default();
-    let bind: std::net::SocketAddr = "0.0.0.0:0".parse().unwrap();
+    let bind: std::net::SocketAddr = "[::]:0".parse().unwrap();
     let endpoint = wzp_transport::create_endpoint(bind, None).map_err(|e| format!("{e}"))?;
     let client_cfg = wzp_transport::client_config();
 
@@ -914,7 +914,13 @@ fn do_register_signal(
     // endpoints, which made MikroTik look symmetric and broke direct
     // P2P because the advertised reflex port was not the listening
     // port.
-    let bind: std::net::SocketAddr = "0.0.0.0:0".parse().unwrap();
+    // [::]:0 = dual-stack socket — handles IPv4 (via ::ffff:x.x.x.x
+    // mapped addresses) AND native IPv6 on one socket. Critical for
+    // Phase 5.5 ICE host candidates: without dual-stack, the IPv6
+    // candidates advertised in DirectCallOffer/Answer are dead on
+    // arrival — the Dialer can't send to them and the Acceptor can't
+    // receive from them.
+    let bind: std::net::SocketAddr = "[::]:0".parse().unwrap();
     let (server_cfg, _cert_der) = wzp_transport::server_config();
     let endpoint = wzp_transport::create_endpoint(bind, Some(server_cfg))
         .map_err(|e| format!("{e}"))?;