feat: add Silero VAD toggle to audio pipeline

Integrates @ricky0123/vad-web's MicVAD as an optional voice activity detector
alongside the noise gate. When enabled, the Silero ONNX model classifies each
audio frame as speech or silence; silence frames mute the worklet's output via
a new VAD gate message. VAD is wired into Publisher.ts alongside the existing
noise gate transformer. Vite is configured to copy the worklet bundle, ONNX
model, and ORT WASM files to /vad/ so they're reachable at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/livekit/NoiseGateTransformer.ts

@@ -119,6 +119,11 @@ export class NoiseGateTransformer implements AudioTrackProcessor {
     this.sendParams();
   }
 
+  /** Tell the worklet to open or close the VAD-controlled gate. */
+  public setVADOpen(open: boolean): void {
+    this.workletNode?.port.postMessage({ type: "vad-gate", open });
+  }
+
   private sendParams(): void {
     if (!this.workletNode) return;
     log.debug("sendParams:", this.params);