Add hello-snapshot UIWorker example

Smallest UIWorker demo: a voice LLM in the main pipeline delegates
screen-relevant utterances to a UIWorker via a respond job; the UIWorker
auto-injects the current <ui_state> and answers grounded in what's on screen.
Includes a vanilla-JS client that streams accessibility snapshots over RTVI.

examples/multi-worker/ui-worker/hello-snapshot/client/main.js

@@ -0,0 +1,110 @@
+/**
+ * Hello UIAgent — vanilla JS client.
+ *
+ * Wires three pieces of the SDK end to end:
+ *   1. PipecatClient + SmallWebRTCTransport for the voice session.
+ *   2. PipecatClient-managed accessibility snapshot streaming on
+ *      every meaningful change (DOM mutations, focus, scroll-end,
+ *      resize, visibility, selection).
+ *
+ * The agent has no tools — the snapshot is the entire input. The
+ * server's ``UIAgent`` auto-injects the latest ``<ui_state>`` block
+ * into the LLM context at the start of every turn, so the agent
+ * always answers grounded in what's currently on screen.
+ */
+
+import { PipecatClient, RTVIEvent } from "@pipecat-ai/client-js";
+import { SmallWebRTCTransport } from "@pipecat-ai/small-webrtc-transport";
+
+const BOT_URL = "http://localhost:7860/api/offer";
+
+const connectButton = document.getElementById("connect");
+const status = document.getElementById("status");
+const botAudio = document.getElementById("bot-audio");
+
+let client;
+
+function setStatus(text, autoHideMs = 0) {
+  status.textContent = text;
+  status.dataset.show = text ? "1" : "0";
+  if (text && autoHideMs > 0) {
+    setTimeout(() => {
+      if (status.textContent === text) status.dataset.show = "0";
+    }, autoHideMs);
+  }
+}
+
+async function connect() {
+  connectButton.disabled = true;
+  setStatus("Connecting…");
+
+  // 1. Construct the Pipecat client with the WebRTC transport.
+  client = new PipecatClient({
+    transport: new SmallWebRTCTransport(),
+    enableMic: true,
+    enableCam: false,
+  });
+
+  client.on(RTVIEvent.BotConnected, () => setStatus("Bot connected", 1500));
+  client.on(RTVIEvent.Disconnected, () => {
+    setStatus("Disconnected", 2000);
+    connectButton.dataset.state = "";
+    connectButton.textContent = "Connect";
+    connectButton.disabled = false;
+    teardownUI();
+  });
+
+  // Pipe the bot's audio track into the <audio> sink so the user
+  // hears it. Without this, the WebRTC track is alive but never
+  // routed to a playback element. The React kit ships
+  // `PipecatClientAudio` to do the same thing — this is the vanilla
+  // equivalent.
+  client.on(RTVIEvent.TrackStarted, (track, participant) => {
+    if (track.kind !== "audio") return;
+    if (participant?.local) return;
+    botAudio.srcObject = new MediaStream([track]);
+  });
+
+  // 3. Connect to the bot.
+  try {
+    await client.connect({ webrtcUrl: BOT_URL });
+    // 2. Start the managed snapshot stream once the RTVI transport is ready.
+    client.startUISnapshotStream();
+    connectButton.dataset.state = "connected";
+    connectButton.textContent = "Disconnect";
+    connectButton.disabled = false;
+    setStatus("Connected. Try asking the agent what's on screen.", 4000);
+  } catch (err) {
+    console.error("Connect failed:", err);
+    setStatus(`Connect failed: ${err.message ?? err}`, 4000);
+    teardownUI();
+    connectButton.disabled = false;
+  }
+}
+
+async function disconnect() {
+  connectButton.disabled = true;
+  setStatus("Disconnecting…");
+  try {
+    await client?.disconnect();
+  } finally {
+    teardownUI();
+    connectButton.dataset.state = "";
+    connectButton.textContent = "Connect";
+    connectButton.disabled = false;
+  }
+}
+
+function teardownUI() {
+  client?.stopUISnapshotStream();
+  if (botAudio.srcObject) botAudio.srcObject = null;
+  client = undefined;
+}
+
+connectButton.addEventListener("click", () => {
+  if (connectButton.dataset.state === "connected") {
+    disconnect();
+  } else {
+    connect();
+  }
+});