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/Cargo.toml

@@ -33,11 +33,24 @@ tauri-plugin-global-shortcut = "2"
 tauri-plugin-updater = "2"
 tauri-plugin-window-state = "2"
 
+# LiveKit client SDK — lives behind the `rust-livekit` feature flag so the
+# baseline build stays unaffected while the JS-SDK path is still the
+# default. Pulls libwebrtc-rs which adds ~20MB to the binary and ~5-10min
+# to the first build. Tokio runtime is required; the rest of the crate
+# stays idle when the feature is off.
+livekit = { version = "0.7", default-features = false, features = ["tokio", "rustls-tls-native-roots"], optional = true }
+tokio = { version = "1", features = ["rt-multi-thread", "sync", "macros"], optional = true }
+
 [features]
 # This feature is used for production builds or when `devPath` points to the filesystem
 # and disables specific features relevant to the dev build.
 custom-protocol = ["tauri/custom-protocol"]
 
+# Enable the Rust LiveKit client. Off by default so CI + users stay on the
+# JS-SDK path until the rust bridge reaches feature parity. Turn on via:
+#   cargo build --features rust-livekit
+rust-livekit = ["dep:livekit", "dep:tokio"]
+
 # Release-profile tuned for ChatApp: whole-program LTO + single codegen unit
 # cuts binary size by ~20-30% and trims startup overhead. `strip = "symbols"`
 # removes debug + symbol tables (the updater already signs separately so