With multiple Horizon workers, ingest jobs for one entity can finish out of
order and overwrite newer state with older. The listener now stamps each message
with a µs receive time (observed_at); IngestShellyMessage applies state only when
the incoming message is newer (race-safe via a conditional update + unique guard),
and broadcasts only when applied. Added a feature test for the ordering guard.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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