fix(audio,ui): address PR #27 review findings

- ios_voice_unit.rs:856-868: Revert iOS render callback from blocking
  lock() back to try_lock() with silence-on-contention (WouldBlock
  branch increments callback_xrun stat and returns the pre-zeroed
  scratch buffer). Blocking lock() inside the CoreAudio HAL render
  callback can stall the realtime IO thread when the decode task on
  engine.rs:1309 holds the same AudioHandler Mutex, re-introducing
  the underrun pattern this codebase already fixed elsewhere.

- ios_voice_unit.rs:782: Preallocate scratch_stereo to Apple's VPIO
  MaximumFramesPerSlice (4096 frames * 2 channels = 8192 f32) at
  setup time, so the realtime render callback never grows the Vec
  via resize(). The defensive 'len() < needed' branch is kept for
  the (impossible) case that the audio unit later raises max frames.

- main.dart:52-56: Gate _showAudioDebugOverlay behind kDebugMode &&
  _isMacOS so the internal audio stats panel does not ship in
  release builds. kDebugMode is a Dart compile-time const, so the
  overlay subtree is tree-shaken out of release/profile binaries.

- Rebuilt macOS chanora_bridge.framework binary (universal arm64 +
  x86_64) from the fixed source with the new CARGO_PROFILE_RELEASE_*
  env vars (DWARF preserved for dsymutil). install_name patched back
  to @rpath/chanora_bridge.framework/Versions/A/chanora_bridge.

  Verified:
    cargo test -p chanora_audio --lib: 129 passed, 0 failed
    cargo check -p chanora_audio --target aarch64-apple-ios: clean
    dart analyze lib/main.dart: no issues
    xcrun lipo -archs: x86_64 arm64
    xcrun otool -D: @rpath install_name preserved

apps/chanora_flutter/lib/main.dart

@@ -49,7 +49,11 @@ import 'widgets/voice_settings.dart';
 import 'package:share_plus/share_plus.dart';
 
 bool get _isMacOS => !kIsWeb && Platform.isMacOS;
-bool get _showAudioDebugOverlay => _isMacOS;
+// Debug-only overlay. `kDebugMode` is a compile-time const in
+// release builds (= false), so the overlay subtree is tree-shaken
+// out of release/profile binaries entirely — release users never
+// see internal audio stats and we don't pay the render cost.
+bool get _showAudioDebugOverlay => kDebugMode && _isMacOS;
 
 const Color _appSurfaceColor = Color(0xFFFFFBFE);
 

crates/chanora_audio/src/ios_voice_unit.rs

@@ -779,7 +779,15 @@ impl IosVoiceUnit {
         // callback's actual output channels. Same as Linux/SDL,
         // just stereo-f32 -> interleaved-i16 converted at the
         // boundary.
-        let mut scratch_stereo: Vec<f32> = Vec::with_capacity(2048);
+        // Preallocate to Apple's VPIO MaximumFramesPerSlice default
+        // (4096 frames) * 2 (stereo) = 8192 f32. The render callback
+        // is realtime; growing this Vec inside the callback would
+        // allocate on the audio thread and risk an underrun. We
+        // never resize below `needed` after this point — the `len()
+        // < needed` branch in the callback is a defensive no-op for
+        // the impossible case where the audio unit later raises
+        // MaximumFramesPerSlice beyond our preallocation.
+        let mut scratch_stereo: Vec<f32> = vec![0.0; 4096 * 2];
         #[cfg(target_os = "ios")]
         let handler_for_render = params.handler.clone();
         #[cfg(target_os = "macos")]
@@ -846,11 +854,15 @@ impl IosVoiceUnit {
                 let _removed = handler_for_render.fill_buffer(&mut scratch_stereo[..needed]);
             }
             #[cfg(target_os = "ios")]
-            match handler_for_render.lock() {
+            match handler_for_render.try_lock() {
                 Ok(mut h) => {
                     let _removed = h.fill_buffer(&mut scratch_stereo[..needed]);
                 }
-                Err(e) => {
+                Err(std::sync::TryLockError::WouldBlock) => {
+                    audio_processing_stats_for_render.increment_callback_xrun();
+                    // scratch_stereo is already zeroed above.
+                }
+                Err(std::sync::TryLockError::Poisoned(e)) => {
                     // Never panic on the realtime IO thread.
                     warn!(target: "chanora_audio", "AudioHandler mutex poisoned: {e}");
                 }