diag(ios,audio): log AVAudioSession state + route changes around picker overrides

Add structured NSLog instrumentation to AppDelegate.swift and debugPrint
chains in voice_compact.dart::_AudioOutputPickerSheetState so we can
correlate user picker taps with what iOS actually does to the route.

Three diagnostic streams:

* 'chanora.session[<tag>]' from Swift — full session snapshot (category,
  mode, sampleRate, ioBufferDuration, current route inputs+outputs,
  preferredInput) emitted on every setActive and every
  AVAudioSession.routeChangeNotification with the reason decoded
  (override / routeConfigurationChange / newDeviceAvailable / etc).
* 'chanora.route[<tag>]' from Dart — current route's inputs+outputs
  emitted before/after every overrideOutputAudioPort or
  setPreferredInput call, plus a delayed re-check at +250 ms to detect
  silent reverts.
* Existing 'chanora: ...' debugPrint lines from the picker now include
  the OK case (override returned, setPreferredInput returned) so we see
  a positive signal in the log when the API didn't throw.

Used to root-cause the 'speaker selector not working' issue: the
hypothesis is that cpal's RemoteIO AudioUnit reacts to its own format
configuration notifications by triggering routeConfigurationChange
that reverts our Dart-side override. The logs will confirm or deny
this — if we see 'chanora.session[routeChange.override] out=Speaker'
followed by 'chanora.session[routeChange.routeConfigurationChange]
out=Receiver' within a few hundred ms, that's the smoking gun.

Pure diagnostic commit. No behavioural change. Logs are NSLog +
debugPrint so they appear in Xcode console / 'flutter logs' / the
device log via Console.app or 'devicectl device log'.

apps/chanora_flutter/ios/Runner/AppDelegate.swift

@@ -85,6 +85,20 @@ import AVFoundation
       object: nil
     )
 
+    // Diagnostic: subscribe to every AVAudioSession route change so
+    // we can correlate user picker actions with what iOS actually
+    // does to the route. The reason printed here ("override",
+    // "newDeviceAvailable", "categoryChange", etc.) tells us
+    // whether our Dart-side override was honoured or silently
+    // reverted by another component (e.g. cpal's RemoteIO unit
+    // reacting to its own configuration change).
+    NotificationCenter.default.addObserver(
+      self,
+      selector: #selector(handleRouteChange(_:)),
+      name: AVAudioSession.routeChangeNotification,
+      object: nil
+    )
+
     // Request microphone access on first launch rather than waiting
     // for the user's first voice-channel join. The latter is
     // surprising: the user has only tapped "connect to server" and
@@ -123,11 +137,69 @@ import AVFoundation
     do {
       try AVAudioSession.sharedInstance().setActive(true, options: [])
       NSLog("chanora_flutter: AVAudioSession activated on foreground")
+      logSessionState(tag: "didBecomeActive")
     } catch {
       NSLog("chanora_flutter: AVAudioSession setActive failed: \(error)")
     }
   }
 
+  /// Log the full AVAudioSession state with a diagnostic tag. Used
+  /// after every state transition (setActive, route change) so we
+  /// can correlate user-perceived audio bugs with what iOS thinks
+  /// the session looks like. Output is parseable by grep
+  /// `chanora.session\[`.
+  private func logSessionState(tag: String) {
+    let s = AVAudioSession.sharedInstance()
+    let route = s.currentRoute
+    let outs = route.outputs.map { "\($0.portType.rawValue)/\($0.portName)" }.joined(separator: ",")
+    let ins = route.inputs.map { "\($0.portType.rawValue)/\($0.portName)" }.joined(separator: ",")
+    let preferredInput = s.preferredInput?.portName ?? "<nil>"
+    NSLog("""
+      chanora.session[\(tag)] \
+      cat=\(s.category.rawValue) \
+      mode=\(s.mode.rawValue) \
+      sr=\(s.sampleRate) \
+      ioBuf=\(String(format: "%.4f", s.ioBufferDuration)) \
+      out=[\(outs)] in=[\(ins)] \
+      preferredInput=\(preferredInput)
+      """)
+  }
+
+  /// Called by `routeChangeNotification`. iOS posts this whenever
+  /// the route is reconfigured for any reason — user toggles
+  /// Bluetooth, plugs in headphones, our `overrideOutputAudioPort`
+  /// call, OR (the suspected bug here) when cpal's RemoteIO
+  /// AudioUnit reacts to its own format change and resets the
+  /// route. The `reason` field tells us which case it is:
+  ///
+  ///   * `.override`           : our Dart-side override took effect.
+  ///   * `.routeConfigurationChange` : something else (cpal?)
+  ///                              triggered an internal reconfig.
+  ///   * `.newDeviceAvailable` / `.oldDeviceUnavailable` :
+  ///                              user hardware change.
+  ///   * `.categoryChange`     : someone (us or another app) set a
+  ///                              new category.
+  @objc private func handleRouteChange(_ note: Notification) {
+    guard let reasonValue = note.userInfo?[AVAudioSessionRouteChangeReasonKey] as? UInt,
+          let reason = AVAudioSession.RouteChangeReason(rawValue: reasonValue) else {
+      NSLog("chanora.routeChange[unknown-reason]")
+      return
+    }
+    let reasonName: String
+    switch reason {
+    case .unknown: reasonName = "unknown"
+    case .newDeviceAvailable: reasonName = "newDeviceAvailable"
+    case .oldDeviceUnavailable: reasonName = "oldDeviceUnavailable"
+    case .categoryChange: reasonName = "categoryChange"
+    case .override: reasonName = "override"
+    case .wakeFromSleep: reasonName = "wakeFromSleep"
+    case .noSuitableRouteForCategory: reasonName = "noSuitableRouteForCategory"
+    case .routeConfigurationChange: reasonName = "routeConfigurationChange"
+    @unknown default: reasonName = "default(\(reasonValue))"
+    }
+    logSessionState(tag: "routeChange.\(reasonName)")
+  }
+
   func didInitializeImplicitFlutterEngine(_ engineBridge: FlutterImplicitEngineBridge) {
     GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
   }