docs(audio): document parseBridgeAudioRoute case-sensitivity contract

Adds a doc comment to parseBridgeAudioRoute clarifying that both
iOS and Android producers (IOSAudioLifecycleController.classifyDevice
and AndroidAudioLifecycleController.classifyDevice) emit exact
PascalCase strings.

Case variants (USB_HEADSET, usb_headset, UsbHeadphone) fall through
to unknown by design. This is a silent failure mode worth documenting
so future changes to either platform classifier are paired with a
parser update.

Per PR #28 review feedback.

apps/chanora_flutter/lib/services/audio_lifecycle_service.dart

@@ -7,6 +7,20 @@ import '../src/rust/api.dart' as rust;
 const iosAudioLifecycleChannelName = 'chanora/ios_audio_lifecycle';
 const androidAudioLifecycleChannelName = 'chanora/android_audio_lifecycle';
 
+/// Parses a platform-channel route string into a [rust.BridgeAudioRoute].
+///
+/// The producer contract is:
+/// - iOS: `IOSAudioLifecycleController.classifyDevice` emits one of
+///   `Earpiece`, `Speaker`, `WiredHeadset`, `BluetoothHfp`, `BluetoothA2dp`,
+///   `Unknown`.
+/// - Android: `AndroidAudioLifecycleController.classifyDevice` emits one of
+///   `Earpiece`, `Speaker`, `WiredHeadset`, `UsbHeadset`, `BluetoothHfp`,
+///   `BluetoothA2dp`, `Hdmi`, `Unknown`.
+///
+/// Both producers emit exact PascalCase strings. Case variants (`USB_HEADSET`,
+/// `usb_headset`, `UsbHeadphone`) are NOT handled and will fall through to
+/// `unknown`. If either platform classifier changes its string contract,
+/// update both producers and this parser together.
 rust.BridgeAudioRoute parseBridgeAudioRoute(String value) {
   switch (value) {
     case 'Earpiece':