fix(android): playout Usage::Media + relay CallSetup advertises real IP

Three real bugs, one smoke-test session's worth of progress.

1. RELAY: wrong advertised addr in CallSetup
   The direct-call CallSetup computed `relay_addr = addr.ip()` where
   `addr = connection.remote_address()` — i.e. the CLIENT'S IP, not the
   relay's. So the relay was telling both parties "the call room is at
   the answerer's IP:4433", which meant each client dialed either the
   other client (no server listening) or themselves. Both endpoint.connect
   calls hung forever and the call never happened.
   Fix: compute the relay's own advertised IP once at startup. If the
   listen addr is 0.0.0.0, probe the primary outbound interface via the
   classic UDP-bind-and-connect(8.8.8.8:80) trick to discover the LAN
   IP the OS would use to reach external hosts. Thread the resulting
   advertised_addr_str into the CallSetup sender for both parties.

2. RELAY: accept loop serialized QUIC handshakes
   Previously the main accept loop called `wzp_transport::accept` which
   did both `endpoint.accept().await` AND `incoming.await` (the server-
   side QUIC handshake). A single slow handshake therefore blocked every
   subsequent client from being accepted. Unroll the helper here and
   move `incoming.await` into the per-connection spawned task, so every
   handshake runs in parallel. Also log "accept queue: new Incoming",
   "QUIC handshake complete", and "QUIC handshake failed" so we can tell
   immediately whether a client's packets are reaching the relay at all.

3. ANDROID: playout was routed to the silent in-call stream
   The Oboe playout stream was configured with Usage::VoiceCommunication,
   which routes to the Android in-call earpiece stream. That stream is
   silent unless the Activity has called AudioManager.setMode(
   IN_COMMUNICATION) and, even then, only the earpiece/BT headset get
   audio (not the loud speaker). Result: android→mac calls worked
   because mac had a normal media output, but mac→android calls were
   silent even though packets flowed through the relay just fine.
   Switch to Usage::Media + ContentType::Speech so Oboe routes to the
   loud speaker and uses the media volume slider. A later polish step
   will wire setMode + setSpeakerphoneOn from MainActivity.kt so we can
   go back to VoiceCommunication for AEC and proximity-sensor routing.

Plus: heartbeat tracing every 2s in the send/recv tasks — frames_sent,
last_rms, last_pkt_bytes, short_reads on the send side; decoded_frames,
last_decode_n, last_written, decode_errs on the recv side. Will make the
next "no sound" regression trivial to localize.

crates/wzp-native/cpp/oboe_bridge.cpp

@@ -195,6 +195,16 @@ int wzp_oboe_start(const WzpOboeConfig* config, const WzpOboeRings* rings) {
     }
 
     // Build playout stream
+    //
+    // Usage::Media (NOT VoiceCommunication) routes to the media audio
+    // stream which plays through the loud speaker and uses the media
+    // volume slider. VoiceCommunication routes to the in-call earpiece
+    // stream which is silent unless AudioManager.setMode(IN_COMMUNICATION)
+    // has been called from the Activity, and even then only the earpiece
+    // (or a bluetooth headset) gets audio by default. For a debug-friendly
+    // smoke test we want loud speaker by default. A future polish step
+    // will wire setMode + setSpeakerphoneOn from MainActivity.kt so we
+    // can switch back to VoiceCommunication (for AEC benefits etc).
     oboe::AudioStreamBuilder playoutBuilder;
     playoutBuilder.setDirection(oboe::Direction::Output)
         ->setPerformanceMode(oboe::PerformanceMode::LowLatency)
@@ -203,7 +213,8 @@ int wzp_oboe_start(const WzpOboeConfig* config, const WzpOboeRings* rings) {
         ->setChannelCount(config->channel_count)
         ->setSampleRate(config->sample_rate)
         ->setFramesPerDataCallback(config->frames_per_burst)
-        ->setUsage(oboe::Usage::VoiceCommunication)
+        ->setUsage(oboe::Usage::Media)
+        ->setContentType(oboe::ContentType::Speech)
         ->setDataCallback(&g_playout_cb);
 
     result = playoutBuilder.openStream(g_playout_stream);