Prototype `kafka_event_aggregator`

[Tom-Willemsen]

Many unordered streams in, one ordered stream out.

TTL-based.

As discussed with DSG 9/2/2026.

[Tom-Willemsen]

c.f. https://github.com/ISISNeutronMuon/digital-muon-pipeline/tree/main/digitiser-aggregator