Hardware-test MQTT telemetry publish path

[TalkingJupiter]

Context

The firmware has moved from UDP telemetry to MQTT publish using PubSubClient. Local builds pass for both esp32_a and esp32_b, but the MQTT path still needs validation on real ESP32 + W5500 hardware against Mosquitto.

Scope

Verify that telemetry is published to repacss/devices/<mac>/telemetry.

Suggested validation

• Start a broker subscriber: mosquitto_sub -h localhost -p 1883 -t 'repacss/#' -v
• Flash one controller and confirm MQTT connection logs appear on serial.
• Confirm telemetry payloads arrive every TELEMETRY_SEND_MS from Controller A.
• Confirm Controller B stays quiet while A is healthy.
• Confirm Controller B publishes after heartbeat failover.

Acceptance criteria

• Telemetry appears on repacss/devices/<mac>/telemetry.
• Payload JSON includes the ESP32 MAC and expected heartbeat/sensor/failover fields.
• A/B failover behavior still prevents duplicate telemetry streams during normal operation.