docs(rust): add doc comments to delta enums and fix dead_code warnings (#24)

* docs(rust): add doc comments to delta enums and fix dead_code warnings

Add missing documentation to ProtocolDelta, CoreDelta, and BridgeDelta
enum variants and their struct fields across the protocol, core, and
bridge crates. Document the ChannelId::ROOT constant and the
take_delta_rx adapter method.

Fix dead_code warnings:
- keyring_disabled: add #[cfg] gate matching its callers
- snapshot_signature: add #[cfg(test)] for future test use

* fix(rust): correct `order` field docs to predecessor channel ID, narrow audio engine cfg gates

- Correct `order` field documentation in ProtocolDelta, SessionEvent,
  and BridgeEvent from 'sort order' to 'predecessor channel ID
  (TeamSpeak linked-list ordering hint)' per Copilot review feedback.
- Narrow AudioEngine voice_out_tx, voice_activity_selector, and mic_gain
  cfg gates from ios+macos+android to android-only, since these fields
  are only read from self in android_restart_voice_unit. On iOS/macOS the
  values are passed directly to the voice backend at construction time.

crates/chanora_audio/src/engine.rs

@@ -306,11 +306,11 @@ pub struct AudioEngine {
     audio_processing_config: Arc<Mutex<crate::AudioProcessingConfig>>,
     audio_processing_stats: Arc<crate::SharedAudioProcessingStats>,
     audio_handler: Arc<Mutex<AudioHandler<SessionAudioId>>>,
-    #[cfg(any(target_os = "ios", target_os = "macos", target_os = "android"))]
+    #[cfg(target_os = "android")]
     voice_out_tx: mpsc::Sender<OutPacket>,
-    #[cfg(any(target_os = "ios", target_os = "macos", target_os = "android"))]
+    #[cfg(target_os = "android")]
     voice_activity_selector: Option<Arc<crate::TransmitModeSelector>>,
-    #[cfg(any(target_os = "ios", target_os = "macos", target_os = "android"))]
+    #[cfg(target_os = "android")]
     mic_gain: f32,
     // Streams must be dropped to stop audio. Both are `!Send` because
     // cpal's Stream isn't Send on some backends; we keep them in an
@@ -1308,9 +1308,6 @@ impl AudioEngine {
             audio_processing_config,
             audio_processing_stats,
             audio_handler,
-            voice_out_tx,
-            voice_activity_selector: cfg.voice_activity_selector.clone(),
-            mic_gain: cfg.mic_gain,
             _ios_voice_backend: Mutex::new(Some(ios_voice_backend)),
             shutdown_tx: Some(shutdown_tx),
             capture_active,