Function handle_frame 

async fn handle_frame(
    msg: Message,
    write: &mut SplitSink<WebSocketStream<MaybeTlsStream<TcpStream>>, Message>,
    channel_id: &str,
    target_agent: &Option<String>,
    health: &Arc<Mutex<BridgeHealth>>,
) -> Result<FrameOutcome, String>

Handles one WS frame: ACKs events_api/slash_commands envelopes immediately (before any parsing/routing — spec §2.2’s hard 3-second requirement), then routes events_api payloads to the reactive bus.