android(audio): Usage::VoiceCommunication + MODE_IN_COMMUNICATION, default handset

With da106bd (Usage::Media + MODE_NORMAL) audio works but is always on
the loudspeaker — we want handset as the default with a user-driven
toggle for speaker (and later bluetooth). The right Oboe usage for a
VoIP app is VoiceCommunication, which honours
AudioManager.setSpeakerphoneOn / setBluetoothScoOn for routing.

Bisection across previous builds showed that setAudioApi(AAudio) +
Usage::VoiceCommunication made the playout callback stop draining the
ring after cb#0 (build 8c36fb5 logs). Letting Oboe pick the AudioApi
implicitly keeps the callback alive — 96be740's Media-usage callbacks
fired at steady 50Hz without any explicit setAudioApi. So: keep the
Usage change, DROP the explicit AAudio force.

- oboe_bridge.cpp: Usage::VoiceCommunication, no setAudioApi, no
  ContentType override.
- MainActivity.kt: setMode(MODE_IN_COMMUNICATION) +
  setSpeakerphoneOn(false) = handset default, plus max both
  STREAM_VOICE_CALL and STREAM_MUSIC volumes for belt-and-braces.

Next build will add a JNI-based Tauri command to flip speakerphoneOn
at runtime so the user can toggle handset↔speaker during a call.

crates/wzp-native/cpp/oboe_bridge.cpp

@@ -297,6 +297,20 @@ int wzp_oboe_start(const WzpOboeConfig* config, const WzpOboeRings* rings) {
     // config that at least drove callbacks correctly, plus the
     // Kotlin-side MODE_IN_COMMUNICATION + setSpeakerphoneOn(true)
     // handled in MainActivity.kt to route audio to the loud speaker.
+    // Usage::VoiceCommunication is the correct Oboe usage for a VoIP app
+    // — it respects Android's in-call audio routing and lets
+    // AudioManager.setSpeakerphoneOn/setBluetoothScoOn actually switch
+    // between earpiece, loudspeaker, and Bluetooth headset. Combined with
+    // MODE_IN_COMMUNICATION set from MainActivity.kt and
+    // speakerphoneOn=false by default, this produces handset/earpiece as
+    // the default output.
+    //
+    // IMPORTANT: do NOT add setAudioApi(AAudio) here. Build 8c36fb5 proved
+    // forcing AAudio with Usage::VoiceCommunication makes the playout
+    // callback stop draining the ring after cb#0, even though the stream
+    // opens successfully. Letting Oboe pick the API (which will be AAudio
+    // on API ≥ 27 but via a different codepath) kept callbacks firing in
+    // every other build.
     oboe::AudioStreamBuilder playoutBuilder;
     playoutBuilder.setDirection(oboe::Direction::Output)
         ->setPerformanceMode(oboe::PerformanceMode::LowLatency)
@@ -305,7 +319,7 @@ 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::Media)
+        ->setUsage(oboe::Usage::VoiceCommunication)
         ->setDataCallback(&g_playout_cb);
 
     result = playoutBuilder.openStream(g_playout_stream);

desktop/src-tauri/gen/android/app/src/main/java/com/wzp/desktop/MainActivity.kt

@@ -57,19 +57,20 @@ class MainActivity : TauriActivity() {
   }
 
   /**
-   * Max out STREAM_MUSIC so the Oboe playout stream (opened with
-   * Usage::Media, which routes to STREAM_MUSIC) is actually audible.
+   * Put the phone into VoIP call mode with handset (earpiece) as the
+   * default output. The Oboe playout stream is opened with
+   * Usage::VoiceCommunication which honours this routing, so:
    *
-   * DELIBERATELY does NOT call setMode(IN_COMMUNICATION) or
-   * setSpeakerphoneOn: build 8c36fb5 confirmed that combining those with
-   * Usage::Media OR with Usage::VoiceCommunication (both tried) broke the
-   * Oboe playout callback entirely — the ring filled once at startup and
-   * Oboe stopped draining it. Keeping audio mode in MODE_NORMAL so the
-   * Media stream follows the normal speaker-output path, controlled by
-   * the media volume slider.
+   *   MODE_IN_COMMUNICATION + speakerphoneOn=false  → earpiece (handset)
+   *   MODE_IN_COMMUNICATION + speakerphoneOn=true   → loudspeaker
+   *   MODE_IN_COMMUNICATION + bluetoothScoOn=true   → bluetooth headset
    *
-   * A polished version of the app will setMode/setSpeakerphoneOn on a
-   * per-call basis once we've figured out the correct combo with AAudio.
+   * The speaker/handset/BT toggle itself is wired up via the Tauri
+   * command `set_speakerphone(on)` in a follow-up build. For now the
+   * default is handset, matching the user's stated preference.
+   *
+   * STREAM_VOICE_CALL volume is cranked to max since the in-call volume
+   * slider is separate from media volume on most devices.
    */
   private fun configureAudioForCall() {
     try {
@@ -80,12 +81,19 @@ class MainActivity : TauriActivity() {
         "musicVol=${am.getStreamVolume(AudioManager.STREAM_MUSIC)}/" +
         "${am.getStreamMaxVolume(AudioManager.STREAM_MUSIC)}")
 
-      // Crank media volume to max — STREAM_MUSIC is what Usage::Media
-      // plays through. User can adjust with hardware volume buttons.
+      am.mode = AudioManager.MODE_IN_COMMUNICATION
+      am.isSpeakerphoneOn = false   // default: handset / earpiece
+
+      // Crank both voice-call and music volumes so nothing silent slips
+      // through regardless of which stream actually ends up driving.
+      val maxVoice = am.getStreamMaxVolume(AudioManager.STREAM_VOICE_CALL)
+      am.setStreamVolume(AudioManager.STREAM_VOICE_CALL, maxVoice, 0)
       val maxMusic = am.getStreamMaxVolume(AudioManager.STREAM_MUSIC)
       am.setStreamVolume(AudioManager.STREAM_MUSIC, maxMusic, 0)
 
-      Log.i(TAG, "audio state after: mode=${am.mode} musicVol=${am.getStreamVolume(AudioManager.STREAM_MUSIC)}/$maxMusic")
+      Log.i(TAG, "audio state after: mode=${am.mode} speaker=${am.isSpeakerphoneOn} " +
+        "voiceVol=${am.getStreamVolume(AudioManager.STREAM_VOICE_CALL)}/$maxVoice " +
+        "musicVol=${am.getStreamVolume(AudioManager.STREAM_MUSIC)}/$maxMusic")
     } catch (e: Throwable) {
       Log.e(TAG, "configureAudioForCall failed: ${e.message}", e)
     }