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_protocol/src/adapter.rs

@@ -471,6 +471,9 @@ impl ProtocolClient {
         }
     }
 
+    /// Takes ownership of the delta receiver channel. Returns `None` if already
+    /// taken. Must be called exactly once during initialization to subscribe to
+    /// incremental state changes.
     pub fn take_delta_rx(&self) -> Option<mpsc::Receiver<ProtocolDelta>> {
         self.delta_rx.lock().ok().and_then(|mut g| g.take())
     }