feat(examples): ESP32 WiFi + MQTT (PubSubClient) gallery example

Adds a self-contained ESP32 networking example for the /examples gallery
(addresses feature request #115). The sketch joins the emulator AP
"Velxio-GUEST", connects to a public MQTT broker (broker.hivemq.com:1883),
then publishes to its own topic and subscribes to it so each message
round-trips through the broker and toggles GPIO2 -- no external client or
local broker needed; just open the Serial Monitor.

Verified end to end in QEMU: WiFi associates (IP 192.168.4.15), DNS resolves
and outbound TCP to :1883 succeeds via slirp NAT. PubSubClient is auto-
installed via the example's `libraries` field.

frontend/src/data/examples-esp32-mqtt.ts

@@ -0,0 +1,109 @@
+/**
+ * ESP32 WiFi + MQTT gallery example.
+ *
+ * Proves the networking path end to end: the ESP32 joins the emulator's
+ * virtual AP (SSID "Velxio-GUEST"), reaches the internet via QEMU slirp NAT,
+ * and round-trips MQTT messages through a public broker (broker.hivemq.com).
+ *
+ * Self-contained: the sketch publishes to a unique topic AND subscribes to
+ * the same topic, so each message comes back through the broker (toggling
+ * GPIO2). No external client or local broker needed — just open the Serial
+ * Monitor at 115200 and watch TX/RX. Verified in QEMU: WiFi connects, DNS
+ * resolves, TCP to :1883 succeeds.
+ */
+import type { ExampleProject } from './examples';
+
+const ESP32_MQTT_CODE = `// ESP32 + WiFi + MQTT (PubSubClient)
+// Joins the emulator AP "Velxio-GUEST", connects to a public MQTT broker,
+// then publishes to its own topic and subscribes to it, so every message
+// round-trips through the broker and toggles GPIO2. Open the Serial Monitor
+// at 115200. Wire an LED to GPIO2 to watch it blink on each round-trip.
+#include <WiFi.h>
+#include <PubSubClient.h>
+
+const char* WIFI_SSID   = "Velxio-GUEST";   // open AP the emulator advertises
+const char* MQTT_BROKER = "broker.hivemq.com";
+const int   MQTT_PORT   = 1883;
+const int   LED         = 2;
+
+WiFiClient net;
+PubSubClient mqtt(net);
+String topic;  // unique per board so two ESP32s don't collide
+
+void onMessage(char* t, byte* payload, unsigned int len) {
+  String m;
+  for (unsigned int i = 0; i < len; i++) m += (char)payload[i];
+  Serial.printf("RX [%s]: %s\\n", t, m.c_str());
+  digitalWrite(LED, !digitalRead(LED));  // toggle on every round-trip
+}
+
+void connectWiFi() {
+  Serial.printf("WiFi: joining %s ...\\n", WIFI_SSID);
+  WiFi.mode(WIFI_STA);
+  WiFi.begin(WIFI_SSID);
+  while (WiFi.status() != WL_CONNECTED) { delay(300); Serial.print("."); }
+  Serial.printf("\\nWiFi connected, IP %s\\n", WiFi.localIP().toString().c_str());
+}
+
+void connectMQTT() {
+  mqtt.setServer(MQTT_BROKER, MQTT_PORT);
+  mqtt.setCallback(onMessage);
+  while (!mqtt.connected()) {
+    String cid = "velxio-" + String((uint32_t)ESP.getEfuseMac(), HEX);
+    Serial.printf("MQTT: connecting to %s as %s ...\\n", MQTT_BROKER, cid.c_str());
+    if (mqtt.connect(cid.c_str())) {
+      Serial.println("MQTT connected");
+      mqtt.subscribe(topic.c_str());
+      Serial.printf("Subscribed to %s\\n", topic.c_str());
+    } else {
+      Serial.printf("MQTT failed, rc=%d, retry in 2s\\n", mqtt.state());
+      delay(2000);
+    }
+  }
+}
+
+unsigned long lastPub = 0;
+int counter = 0;
+
+void setup() {
+  Serial.begin(115200);
+  pinMode(LED, OUTPUT);
+  delay(500);
+  topic = "velxio/demo/" + String((uint32_t)ESP.getEfuseMac(), HEX);
+  connectWiFi();
+  connectMQTT();
+}
+
+void loop() {
+  if (!mqtt.connected()) connectMQTT();
+  mqtt.loop();
+  if (millis() - lastPub > 2000) {
+    lastPub = millis();
+    String msg = "hello " + String(counter++);
+    mqtt.publish(topic.c_str(), msg.c_str());
+    Serial.printf("TX [%s]: %s\\n", topic.c_str(), msg.c_str());
+  }
+}
+`;
+
+export const esp32MqttExamples: ExampleProject[] = [
+  {
+    id: 'esp32-wifi-mqtt',
+    title: 'ESP32 WiFi + MQTT',
+    description:
+      'Connect an ESP32 to WiFi and a public MQTT broker. The sketch publishes ' +
+      "to its own topic and subscribes to it, so every message round-trips through " +
+      'broker.hivemq.com and toggles GPIO2 — a self-contained test of WiFi and MQTT ' +
+      'with no external setup. Open the Serial Monitor at 115200 to watch it connect ' +
+      'and exchange messages. The emulator advertises an open AP named "Velxio-GUEST".',
+    category: 'communication',
+    difficulty: 'intermediate',
+    boardType: 'esp32',
+    boardFilter: 'esp32',
+    tags: ['esp32', 'wifi', 'mqtt', 'pubsubclient', 'iot', 'network'],
+    libraries: ['PubSubClient'],
+    code: ESP32_MQTT_CODE,
+    components: [],
+    wires: [],
+  },
+];

frontend/src/data/examples.ts

@@ -13,6 +13,7 @@ import { epaperExamples } from './examples-displays-epaper';
 import { retroIntelExamples } from './examples-retro-intel';
 import { robotDesktopExamples } from './examples-robot-desktop';
 import { microsdExamples } from './examples-storage-microsd';
+import { esp32MqttExamples } from './examples-esp32-mqtt';
 
 /** Per-board setup for multi-board examples */
 export interface ExampleBoard {
@@ -9739,6 +9740,7 @@ export const exampleProjects: ExampleProject[] = [
   ...retroIntelExamples,
   ...robotDesktopExamples,
   ...microsdExamples,
+  ...esp32MqttExamples,
 ];
 
 // Get examples by category