feat(outbox): transactional outbox helper — Python core (ADR-0029) #44

Job	Run time
Static analysis (ruff + mypy)	54s
Broker integration (Redis · RabbitMQ · SQS)	1m 17s
Conformance suite in sync	6s
Python 3.11	18s
Python 3.10	18s
Python 3.9	32s
Python 3.13	19s
Python 3.12	22s
CI green	4s
	4m 10s

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