feat: codec selection in settings (Opus / Opus Low / Codec2)

- Settings UI: radio buttons for encode codec selection
- Persisted via SettingsRepository
- Passed through WzpEngine.startCall(profile=) → JNI → Rust CallStartConfig
- Decode always accepts all codecs (per-packet codec_id switch)
- 0 = Opus 24k (GOOD), 1 = Opus 6k (DEGRADED), 2 = Codec2 1.2k (CATASTROPHIC)

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

android/app/src/main/java/com/wzp/ui/call/CallViewModel.kt

@@ -109,6 +109,10 @@ class CallViewModel : ViewModel(), WzpCallback {
     private val _debugRecording = MutableStateFlow(false)
     val debugRecording: StateFlow<Boolean> = _debugRecording.asStateFlow()
 
+    // 0 = Opus (GOOD), 1 = Opus Low (DEGRADED), 2 = Codec2 (CATASTROPHIC)
+    private val _codecChoice = MutableStateFlow(0)
+    val codecChoice: StateFlow<Int> = _codecChoice.asStateFlow()
+
     /** True when a call just ended and debug report can be sent. */
     private val _debugReportAvailable = MutableStateFlow(false)
     val debugReportAvailable: StateFlow<Boolean> = _debugReportAvailable.asStateFlow()
@@ -164,6 +168,7 @@ class CallViewModel : ViewModel(), WzpCallback {
         _seedHex.value = s.getOrCreateSeedHex()
         _aecEnabled.value = s.loadAecEnabled()
         _debugRecording.value = s.loadDebugRecording()
+        _codecChoice.value = s.loadCodecChoice()
         _recentRooms.value = s.loadRecentRooms()
     }
 
@@ -309,6 +314,11 @@ class CallViewModel : ViewModel(), WzpCallback {
         settings?.saveDebugRecording(enabled)
     }
 
+    fun setCodecChoice(choice: Int) {
+        _codecChoice.value = choice
+        settings?.saveCodecChoice(choice)
+    }
+
     /**
      * Resolve DNS hostname to IP address on the Kotlin/Android side,
      * since Rust's DNS resolution may not work on Android.
@@ -406,7 +416,7 @@ class CallViewModel : ViewModel(), WzpCallback {
                     val seed = _seedHex.value
                     val name = _alias.value
                     Log.i(TAG, "startCall: resolved=$relay, alias=$name, calling engine.startCall")
-                    val result = engine?.startCall(relay, room, seedHex = seed, alias = name) ?: -1
+                    val result = engine?.startCall(relay, room, seedHex = seed, alias = name, profile = _codecChoice.value) ?: -1
                     Log.i(TAG, "startCall: engine returned $result")
                     // Only wire up notification callback after engine is running
                     CallService.onStopFromNotification = { stopCall() }

android/app/src/main/java/com/wzp/ui/settings/SettingsScreen.kt

@@ -1,5 +1,6 @@
 package com.wzp.ui.settings
 
+import androidx.compose.foundation.clickable
 import android.content.ClipData
 import android.content.ClipboardManager
 import android.content.Context
@@ -22,6 +23,7 @@ import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.Button
 import androidx.compose.material3.ButtonDefaults
 import androidx.compose.material3.Divider
+import androidx.compose.material3.RadioButton
 import androidx.compose.material3.FilledTonalButton
 import androidx.compose.material3.FilledTonalIconButton
 import androidx.compose.material3.IconButtonDefaults
@@ -241,6 +243,35 @@ fun SettingsScreen(
                 )
             }
 
+            Spacer(modifier = Modifier.height(12.dp))
+
+            // Codec selection
+            val codecNames = listOf("Opus 24k (Best)", "Opus 6k (Low BW)", "Codec2 1.2k (Minimal)")
+            val currentCodec by viewModel.codecChoice.collectAsState()
+            Text("Encode Codec", style = MaterialTheme.typography.bodyMedium)
+            Text(
+                text = "Decode always accepts all codecs",
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant
+            )
+            Spacer(modifier = Modifier.height(4.dp))
+            codecNames.forEachIndexed { idx, name ->
+                Row(
+                    verticalAlignment = Alignment.CenterVertically,
+                    modifier = Modifier
+                        .fillMaxWidth()
+                        .clickable { viewModel.setCodecChoice(idx) }
+                        .padding(vertical = 4.dp)
+                ) {
+                    RadioButton(
+                        selected = currentCodec == idx,
+                        onClick = { viewModel.setCodecChoice(idx) }
+                    )
+                    Spacer(modifier = Modifier.width(8.dp))
+                    Text(name, style = MaterialTheme.typography.bodyMedium)
+                }
+            }
+
             Spacer(modifier = Modifier.height(24.dp))
             Divider()
             Spacer(modifier = Modifier.height(16.dp))