feat(audio,ios): AVAudioSession PlayAndRecord+voiceChat in AppDelegate

iOS AVAudioSession must be configured BEFORE Flutter starts its
audio pipeline; the canonical place is application(_:didFinishLaunching\
WithOptions:) in AppDelegate.swift. This commit:

apps/chanora_flutter/ios/Runner/AppDelegate.swift:
  * import AVFoundation
  * In application(_:didFinishLaunchingWithOptions:), call
    AVAudioSession.sharedInstance().setCategory(.playAndRecord,
      mode: .voiceChat,
      options: [.defaultToSpeaker, .allowBluetooth, .allowBluetoothA2DP])
    followed by setActive(true). Failures are NSLogged but do not
    block app launch — cpal's CoreAudio backend will still come up
    against the default iOS routing.

  This shape:
   * routes the receiver/speaker like a phone call (.playAndRecord +
     .voiceChat),
   * engages on-device AEC / NS where supported,
   * defaults to speaker so users don't have to hold the phone to
     their ear,
   * permits Bluetooth headsets (AirPods et al. just work).

crates/chanora_audio/src/engine.rs:
  * Replace the iOS engine-start placeholder log line ('binding
    pending — Chanora iOS audio is documented-only for Beta') with
    an honest acknowledgment that the AVAudioSession configuration
    lives Swift-side. The Rust engine acknowledges the request, then
    cpal opens its CoreAudio streams against the session.

iOS-only Rust code is #[cfg(target_os = "ios")]-gated so this commit
is no-op on every other platform.

SRS-197: iOS/macOS audio routing contract. DEC-025: iOS officially
in scope for P0 (Focused PTT only — Apple's sandbox model has no
global PTT analogue).

crates/chanora_audio/src/engine.rs

@@ -261,9 +261,25 @@ impl AudioEngine {
         #[cfg(target_os = "ios")]
         {
             if cfg.mobile_voice_preset {
+                // iOS AVAudioSession configuration is performed
+                // Swift-side in `apps/chanora_flutter/ios/Runner/
+                // AppDelegate.swift::application(_:didFinishLaunching\
+                // WithOptions:)` BEFORE Flutter starts its audio
+                // pipeline. The category/mode set there
+                // (`.playAndRecord` + `.voiceChat`,
+                // `defaultToSpeaker | allowBluetooth |
+                // allowBluetoothA2DP`) is the recommended iOS shape
+                // for voice clients and engages on-device AEC / NS
+                // routing where supported. cpal's CoreAudio
+                // backend then opens its streams against that
+                // session and inherits the routing. Logging here
+                // just records that the engine-start path
+                // acknowledges the request; the actual session
+                // mutation lives in Swift because it must happen
+                // before Dart loads.
                 info!(
                     target: "chanora_audio",
-                    "ios: voice-chat session mode requested (binding pending — Chanora iOS audio is documented-only for Beta)"
+                    "ios: voice-chat session mode requested — AVAudioSession configured in AppDelegate"
                 );
             }
         }