feat(reflect): QUIC-native NAT reflection ("STUN for QUIC") — Phase 1

Lets a client ask its registered relay "what IP:port do you see for
me?" over the existing TLS-authenticated signal channel, returning
the client's server-reflexive address as a SocketAddr. Replaces the
need for a classic STUN deployment and becomes the bootstrap step
for future P2P hole-punching: once both peers know their own reflex
addrs, they can advertise them in DirectCallOffer and attempt a
direct QUIC handshake to each other.

Wire protocol (wzp-proto):
- SignalMessage::Reflect — unit variant, client -> relay
- SignalMessage::ReflectResponse { observed_addr: String } — relay -> client
- JSON-serde, appended at end of enum: zero ordinal concerns,
  backward compat with pre-Phase-1 relays by construction (older
  relays log "unexpected message" and drop; newer clients time out
  cleanly within 1s).

Relay handler (wzp-relay/src/main.rs, signal loop):
- New match arm next to Ping reuses the already-bound `addr` from
  connection.remote_address() and replies with observed_addr as a
  string. debug!-level log on success, warn!-level on send failure.

Client side (desktop/src-tauri/src/lib.rs):
- SignalState gains pending_reflect: Option<oneshot::Sender<SocketAddr>>.
- get_reflected_address Tauri command installs the oneshot before
  sending Reflect and awaits it with a 1s timeout; cleans up on
  every exit path (send failure, timeout, parse error).
- recv loop's new ReflectResponse arm fires the pending sender or
  emits a debug log for unsolicited responses — never crashes the
  loop on malformed input.
- Integrated into invoke_handler! alongside the other signal
  commands.

UI (desktop/index.html + src/main.ts):
- New "Network" section in settings panel with a "Detect" button
  that displays the reflected address or a categorized warning
  ("register first" / "relay does not support reflection" / error).

Tests (crates/wzp-relay/tests/reflect.rs — 3 new, all passing):
- reflect_happy_path: client on loopback gets back 127.0.0.1:<its own port>
- reflect_two_clients_distinct_ports: two concurrent clients see
  their own distinct ports, proving per-connection remote_address
- reflect_old_relay_times_out: mock relay that ignores Reflect —
  client times out between 1000-1200ms and does not hang

Also pre-existing test bit-rot unrelated to this PR — fixed so the
full workspace `cargo test` goes green:
- handshake_integration tests in wzp-client, wzp-relay and
  featherchat_compat in wzp-crypto all missed the `alias` field
  addition to CallOffer and the 3-arg form of perform_handshake
  plus 4-tuple return of accept_handshake. Updated to the current
  API surface.

Results:
  cargo test --workspace --exclude wzp-android: 386 passed
  cargo check --workspace: clean
  cargo clippy: no new warnings in touched files

Verification excludes wzp-android because it's dead code on this
branch (Tauri mobile uses wzp-native instead) and can't link -llog
on macOS host — unchanged status quo.

PRD: .taskmaster/docs/prd_reflect_over_quic.txt
Tasks: 39-46 all completed

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

crates/wzp-proto/src/packet.rs

@@ -770,6 +770,29 @@ pub enum SignalMessage {
     CallRinging {
         call_id: String,
     },
+
+    // ── NAT reflection ("STUN for QUIC") ──────────────────────────────
+
+    /// Client → relay: "please tell me the source IP:port you see on
+    /// this connection". A QUIC-native replacement for classic STUN
+    /// that reuses the TLS-authenticated signal channel to the relay
+    /// instead of running a separate UDP reflection service on port
+    /// 3478. The relay answers with `ReflectResponse`.
+    ///
+    /// No payload — the relay already knows which connection the
+    /// request arrived on, and `connection.remote_address()` gives it
+    /// the exact source address (post-NAT) as observed from the
+    /// server side of the TLS session.
+    Reflect,
+
+    /// Relay → client: response to `Reflect`. Carries the socket
+    /// address the relay observes as the client's source for this
+    /// QUIC connection in `SocketAddr::to_string()` form — "a.b.c.d:p"
+    /// for IPv4, "[::1]:p" for IPv6. Clients parse it with
+    /// `SocketAddr::from_str`.
+    ReflectResponse {
+        observed_addr: String,
+    },
 }
 
 /// How the callee responds to a direct call.
@@ -908,6 +931,58 @@ mod tests {
         assert_eq!(packet.quality_report, decoded.quality_report);
     }
 
+    #[test]
+    fn reflect_serialize_roundtrip() {
+        // Reflect is a unit variant — the client sends it with no
+        // payload and the relay answers with the observed source addr.
+        let req = SignalMessage::Reflect;
+        let json = serde_json::to_string(&req).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        assert!(matches!(decoded, SignalMessage::Reflect));
+
+        // ReflectResponse carries a string — exercise both IPv4 and
+        // IPv6 shapes because SocketAddr::to_string uses [::1]:port
+        // for v6 and the client side has to parse that back.
+        for addr in ["192.0.2.17:4433", "[2001:db8::1]:4433", "127.0.0.1:54321"] {
+            let resp = SignalMessage::ReflectResponse {
+                observed_addr: addr.to_string(),
+            };
+            let json = serde_json::to_string(&resp).unwrap();
+            let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+            match decoded {
+                SignalMessage::ReflectResponse { observed_addr } => {
+                    assert_eq!(observed_addr, addr);
+                    // Must parse back to a SocketAddr cleanly.
+                    let _parsed: std::net::SocketAddr = observed_addr.parse()
+                        .expect("observed_addr must parse as SocketAddr");
+                }
+                _ => panic!("wrong variant after roundtrip"),
+            }
+        }
+    }
+
+    #[test]
+    fn reflect_backward_compat_with_existing_variants() {
+        // Adding Reflect/ReflectResponse at the end of the enum must
+        // not break JSON round-tripping of existing variants. Smoke-
+        // test a sample of the pre-existing ones.
+        let cases = vec![
+            SignalMessage::Ping { timestamp_ms: 12345 },
+            SignalMessage::Hold,
+            SignalMessage::Hangup { reason: HangupReason::Normal },
+            SignalMessage::CallRinging { call_id: "abcd".into() },
+        ];
+        for m in cases {
+            let json = serde_json::to_string(&m).unwrap();
+            let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+            // Discriminant equality proves variant tag survived.
+            assert_eq!(
+                std::mem::discriminant(&m),
+                std::mem::discriminant(&decoded)
+            );
+        }
+    }
+
     #[test]
     fn hold_unhold_serialize() {
         let hold = SignalMessage::Hold;