VadFrame: {
    active: boolean;
    channel: string;
    rms: number;
    ts: number;
}

Per-channel, per-frame VAD observation emitted by StreamingTranscriber when running in dual-channel mode. ts is stream-relative milliseconds, derived from the per-channel sample counter — the same reference frame as StreamingWord.start / .end, so per-word lookups need no conversion.