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_storage/src/lib.rs

@@ -628,6 +628,12 @@ fn read_file_dek(path: &Path) -> Result<[u8; 32], StorageError> {
 /// headless / sandboxed environments force the file-fallback path
 /// without poking a real OS keyring (which would either prompt the
 /// user or block on a missing D-Bus session).
+#[cfg(any(
+    target_os = "linux",
+    target_os = "macos",
+    target_os = "windows",
+    target_os = "ios"
+))]
 fn keyring_disabled() -> bool {
     matches!(
         std::env::var("CHANORA_DISABLE_KEYRING").as_deref(),