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);