feat: decouple noise gate and VAD, pre-warm model for instant enable

Noise gate and Silero VAD now work fully independently — the worklet
attaches when either is enabled and bypasses the amplitude gate when
only VAD is on (noiseGateActive flag). SileroVADGate gains a two-phase
lifecycle: init(ctx) loads the ONNX model eagerly when the AudioContext
is first created; start(stream) is then near-instant when the user
enables VAD. stop() pauses without unloading the model so re-enabling
is also instant. VAD checkbox no longer requires the noise gate.

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

src/livekit/NoiseGateTransformer.ts

@@ -11,6 +11,7 @@ import { logger } from "matrix-js-sdk/lib/logger";
 const log = logger.getChild("[NoiseGateTransformer]");
 
 export interface NoiseGateParams {
+  noiseGateActive: boolean;
   threshold: number; // dBFS — gate opens above this, closes below it
   attackMs: number;
   holdMs: number;