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).

apps/chanora_flutter/ios/Runner/AppDelegate.swift

@@ -1,5 +1,6 @@
-import Flutter
 import UIKit
+import Flutter
+import AVFoundation
 
 @main
 @objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
@@ -7,6 +8,33 @@ import UIKit
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
+    // Configure the iOS AVAudioSession for voice chat BEFORE Flutter
+    // starts the audio engine. The category + mode combination tells
+    // iOS to:
+    //   * route via the receiver/speaker like a phone call
+    //     (`.playAndRecord` + `.voiceChat`)
+    //   * engage hardware AEC / NS where the device supports it
+    //   * default the speaker output (so the user doesn't have to
+    //     hold the phone to their ear)
+    //   * permit Bluetooth headsets (so AirPods et al. just work)
+    //
+    // SRS-197 covers the iOS audio routing contract; this is the
+    // matching iOS-side implementation. Failures are logged but do
+    // not block app launch — the audio engine will still come up,
+    // just at the iOS default playback route.
+    do {
+      let session = AVAudioSession.sharedInstance()
+      try session.setCategory(
+        .playAndRecord,
+        mode: .voiceChat,
+        options: [.defaultToSpeaker, .allowBluetooth, .allowBluetoothA2DP]
+      )
+      try session.setActive(true, options: [])
+      NSLog("chanora_flutter: AVAudioSession configured (playAndRecord/voiceChat)")
+    } catch {
+      NSLog("chanora_flutter: AVAudioSession setup failed: \(error)")
+    }
+
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }