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/data/SettingsRepository.kt

@@ -126,6 +126,11 @@ class SettingsRepository(context: Context) {
     fun saveDebugRecording(enabled: Boolean) { prefs.edit().putBoolean(KEY_DEBUG_RECORDING, enabled).apply() }
     fun loadDebugRecording(): Boolean = prefs.getBoolean(KEY_DEBUG_RECORDING, false)
 
+    // --- Codec choice ---
+    // 0 = Opus (GOOD), 1 = Opus Low (DEGRADED), 2 = Codec2 (CATASTROPHIC)
+    fun saveCodecChoice(choice: Int) { prefs.edit().putInt("codec_choice", choice).apply() }
+    fun loadCodecChoice(): Int = prefs.getInt("codec_choice", 0)
+
     // --- Identity seed ---
 
     /**