fix(audio,ios): gate output_underrun on !muted

ios_voice_unit.rs:1033: change the condition from `mix_stats.peak_i16 == 0` to `mix_stats.peak_i16 == 0 && !muted`. When the user mutes the channel via output_muted, the downmix helper fills the output buffer with silence (peak = 0), which previously falsely incremented the output_underrun counter. The mute toggle is intentional silence, not a real underrun.

Note: this does not address the separate false positive where peak_i16 == 0 with output unmuted but no audio incoming (e.g., just joined a channel with no remote speaking). A complete fix would require tracking whether the audio handler actually produced data; deferred to a follow-up.

crates/chanora_audio/src/ios_voice_unit.rs

@@ -861,13 +861,14 @@ impl IosVoiceUnit {
             } else {
                 render_recorder_active = false;
             }
-
             // Track audio-vs-silence for the diagnostic.
             if mix_stats.peak_i16 > 0 {
                 callbacks_with_audio = callbacks_with_audio.wrapping_add(1);
             } else {
-                audio_processing_stats_for_render.increment_output_underrun();
                 callbacks_with_silence = callbacks_with_silence.wrapping_add(1);
+                if !muted {
+                    audio_processing_stats_for_render.increment_output_underrun();
+                }
             }
 
             // Diagnostic sampling.