feat(voice): add iOS VAD runtime support

crates/chanora_audio/src/transmit_mode.rs

@@ -5,33 +5,23 @@
 //! [`chanora_storage::IdentityFileStore`] under the `transmit_mode`
 //! metadata key (default [`TransmitMode::Ptt`]).
 //!
-//! `VoiceActivity` is reserved per DEC-030 — for v1 the
-//! [`crate::transmit_selector::TransmitModeSelector`] treats it
-//! exactly like [`TransmitMode::Continuous`] until a real VAD
-//! implementation lands.
+//! `VoiceActivity` is driven by Rust-owned VAD state in P1.
 
 /// User-visible voice transmit mode.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
 #[repr(u8)]
 pub enum TransmitMode {
     /// Push-to-talk: transmit only while the bound key is held
     /// (with release-tail per SDD-096).
+    #[default]
     Ptt = 0,
     /// Continuous: transmit whenever the user is in a voice
     /// channel and not hard-muted.
     Continuous = 1,
-    /// Voice activity detection. Reserved per DEC-030; v1 behaves
-    /// as [`TransmitMode::Continuous`] until a VAD implementation
-    /// is allocated.
+    /// Voice activity detection.
     VoiceActivity = 2,
 }
 
-impl Default for TransmitMode {
-    fn default() -> Self {
-        Self::Ptt
-    }
-}
-
 impl TransmitMode {
     /// Encode as the persisted single-byte value.
     pub fn as_u8(self) -> u8 {