Respect MQTT fixtures

[MTrab]

Summary:

• add a shared fixture loader that understands both single-document API exports and sequential MQTT dumps and reuse it in the device decoding tests.
• extend tests/test_device_decode.py so every MQTT payload in code-ref/data-samples is validated, and adjust tests/test_fixtures.py plus the docs to mention the extra fixture format.
• make scripts/dump_mapping.py walk both http.json and mqtt.json fixtures so the rendered snapshots match the live MQTT stream while keeping README.md and DATA_MAPPING.md aligned.

Testing:

• bash scripts/prepare_test_fixtures.sh
• source .venv/bin/activate && pytest tests/test_device_decode.py -q
• source .venv/bin/activate && python scripts/dump_mapping.py