feat(livekit): rust SDK scaffold behind rust-livekit feature flag

Phase B.1 of the native-livekit migration. Ships the command surface and
event bridge behind a cargo feature so the default build stays unaffected
while the JS-SDK path keeps running in production.

Rust side
- livekit 0.7 (default tokio runtime, rustls-tls-native-roots) pulled as
  an optional dependency; tokio also optional under the same feature
- Feature `rust-livekit` gates everything — off by default; on via
  `cargo build --features rust-livekit`
- src-tauri/src/livekit_bridge.rs: LivekitState mutex, connect /
  disconnect / send_data commands, event pump for room_state,
  participant_joined, participant_left, data_received
- Mic / camera / screen share commands stubbed with explicit
  "not implemented" errors so JS callers fail loudly rather than
  silently no-op

JS side
- src/lib/nativeLiveKit.ts exposes a NativeRoom class with the same event
  / method shape the CallContext will need, plus a VITE_USE_RUST_LIVEKIT
  flag so the adapter can be swapped once the bridge reaches parity
- isRustLivekitAvailable() gates access at both env + runtime layers

Build impact
- Baseline build unchanged (1s incremental, no new deps pulled)
- Feature build initial: ~10min (libwebrtc download + link)
- Feature build incremental: ~1s
- Binary size with feature: +12-20MB vs baseline

Open questions (documented for the next phase)
- Video-frame rendering bridge remains an upstream gap; livekit-rust
  exposes NativeVideoFrame but no stable path to expose that as a
  MediaStreamTrack inside the WebView
- Audio-only rust path is realistic near-term; full-rust needs either
  upstream video-bridge or a native-overlay render window

apps/desktop/src-tauri/src/lib.rs

@@ -1,5 +1,8 @@
 mod crypto;
 
+#[cfg(feature = "rust-livekit")]
+mod livekit_bridge;
+
 #[cfg(not(any(target_os = "android", target_os = "ios")))]
 use tauri::{
     menu::{Menu, MenuEvent, MenuItem, PredefinedMenuItem},
@@ -49,6 +52,7 @@ fn handle_menu_event(app: &AppHandle, event: MenuEvent) {
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
+    #[cfg(not(feature = "rust-livekit"))]
     let mut builder = tauri::Builder::default()
         .invoke_handler(tauri::generate_handler![
             crypto::crypto_random_bytes,
@@ -61,7 +65,30 @@ pub fn run() {
             crypto::crypto_box_seal_open,
             crypto::crypto_pwhash,
         ])
-        .plugin(tauri_plugin_notification::init())
+        .plugin(tauri_plugin_notification::init());
+
+    #[cfg(feature = "rust-livekit")]
+    let mut builder = tauri::Builder::default()
+        .manage(livekit_bridge::LivekitState::new())
+        .invoke_handler(tauri::generate_handler![
+            crypto::crypto_random_bytes,
+            crypto::crypto_secretbox_encrypt,
+            crypto::crypto_secretbox_decrypt,
+            crypto::crypto_box_keypair,
+            crypto::crypto_box_encrypt,
+            crypto::crypto_box_decrypt,
+            crypto::crypto_box_seal,
+            crypto::crypto_box_seal_open,
+            crypto::crypto_pwhash,
+            livekit_bridge::livekit_connect,
+            livekit_bridge::livekit_disconnect,
+            livekit_bridge::livekit_send_data,
+            livekit_bridge::livekit_set_mic,
+            livekit_bridge::livekit_set_camera,
+        ])
+        .plugin(tauri_plugin_notification::init());
+
+    builder = builder
         .plugin(tauri_plugin_sql::Builder::default().build())
         .plugin(tauri_plugin_fs::init())
         .plugin(