fix(audio,ios): bypass VPIO voice processing for clean playback (rc.8+68)

User report at +67 (.voiceChat mode): playback still 'broken'.
Even with VPIO's Apple-documented session-mode pairing,
its output-side gating chain (echo subtraction + adaptive
noise suppression) chops quiet inter-phoneme content of human
voice. Musicbot signal (loud, ~continuous) survives because it
stays above the gating threshold; speech does not.

Fix: set kAUVoiceIOProperty_BypassVoiceProcessing = 1 on the
unit immediately after EnableIO (before stream format / callbacks
/ initialize). This disables ALL VPIO voice processing \u2014 the
unit becomes effectively a vanilla RemoteIO with mic + speaker
buses. Raw samples pass through both directions.

Trade-off:
* Lost: Apple's hardware AEC + AGC + NS on the mic path. User
  reports current capture is clean already, suggesting their
  test environment (headset? non-speakerphone?) doesn't need
  AEC. If echo loops back when speakerphone is engaged, we'll
  re-evaluate \u2014 either re-enable VPIO selectively for
  echo-prone routes or ship software AEC (DEC-007).
* Gained: playback is no longer gated. Quiet inter-phoneme
  speech content reaches the speaker.

Property setter:
* Constant: kAUVoiceIOProperty_BypassVoiceProcessing = 2100
* Scope: Global, Element: Input (1) per WebRTC's reference iOS
  ADM (voice_processing_audio_unit.mm).
* Value: u32 = 1 (= bypass).
* Soft-fail with warn log if the property is rejected on an
  exotic iOS version (the unit still works, just with VPIO
  defaults).

Build counter 67 -> 68.

apps/chanora_flutter/pubspec.yaml

@@ -16,7 +16,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
 # In Windows, build-name is used as the major, minor, and patch parts
 # of the product and file versions while build-number is used as the build suffix.
-version: 1.0.0-rc.8+67
+version: 1.0.0-rc.8+68
 
 environment:
   sdk: ^3.11.5

crates/chanora_audio/src/ios_voice_unit.rs

@@ -387,6 +387,65 @@ impl IosVoiceUnit {
         )
         .map_err(|e| AudioError::Backend(format!("vpio enable input I/O: {e}")))?;
 
+        // Bypass VPIO's built-in voice processing (AEC + AGC + NS).
+        //
+        // Rationale: with the AVAudioSession in mode .voiceChat
+        // (the documented VPIO pair), VPIO's output-side gating
+        // chain still aggressively gates quiet inter-phoneme
+        // content as "noise". User-reported symptom: musicbot
+        // audio plays cleanly (loud, near-continuous signal),
+        // but human voice with normal 20 dB peak-to-average
+        // ratio sounds broken and unintelligible because the
+        // average-level content gets chopped out between
+        // phonemes.
+        //
+        // Bypassing voice processing turns VPIO into effectively
+        // a vanilla I/O unit \u2014 raw mic samples in, raw
+        // playback out. The trade-offs:
+        //
+        //   * Capture lost: Apple's AEC + AGC + NS on the mic
+        //     input. The user's current setup reports clean
+        //     capture without these (likely using a headset
+        //     where AEC is unnecessary, or not generating
+        //     echo loud enough to matter). If echo loops back
+        //     when on speakerphone, we'll need to either
+        //     re-enable VPIO selectively or ship software AEC
+        //     (DEC-007).
+        //   * Playback gained: signal passes through verbatim.
+        //     Quiet inter-phoneme samples are no longer gated.
+        //
+        // Property constants from Apple's AudioUnitProperties.h
+        // (also exposed via objc2-audio-toolbox):
+        //   kAUVoiceIOProperty_BypassVoiceProcessing = 2100
+        //
+        // Set on (Scope::Global, Element::Input = 1) per WebRTC's
+        // reference iOS audio device manager
+        // (voice_processing_audio_unit.mm). The bypass flag is a
+        // UInt32 with 1 = bypass.
+        const K_AU_VOICE_IO_PROPERTY_BYPASS_VOICE_PROCESSING: u32 = 2100;
+        let bypass: u32 = 1;
+        if let Err(e) = unit.set_property(
+            K_AU_VOICE_IO_PROPERTY_BYPASS_VOICE_PROCESSING,
+            Scope::Global,
+            Element::Input,
+            Some(&bypass),
+        ) {
+            // Soft-fail: if the bypass property is rejected on
+            // an exotic iOS version, log and continue. The unit
+            // is still usable with default voice processing
+            // (with whatever quality issues that brings).
+            warn!(
+                target: "chanora_audio",
+                error = %e,
+                "vpio: BypassVoiceProcessing set failed; continuing with VPIO defaults"
+            );
+        } else {
+            info!(
+                target: "chanora_audio",
+                "vpio: voice processing bypassed (no AEC/AGC/NS) for cleaner playback"
+            );
+        }
+
         // Stream format. Apple's iOS canonical format for
         // AudioUnits is Linear PCM, 16-bit signed integer samples
         // (see "Canonical formats" in the Audio Unit Hosting Guide