MQTT application receipt experiment

Use CPython3.11+ with paho-mqtt==2.1.0 and an existing Mosquitto2.1.2 binary.
Start: mosquitto -c mosquitto-loopback.conf
In another terminal: python door_ack_lab.py receiver --duration 5
In a third terminal: python door_ack_lab.py sender --timeout 1
Tests: MOSQUITTO_BIN=/absolute/path/to/mosquitto python -B test_door_ack.py
Tests launch/terminate their own temporary127.0.0.1 broker on a free high port.
No global service/LAN/GPIO/physical door. The print receipt is volatile, not durable or exactly once.
All host tests are separate from physical and target testing.
