ms-since-epoch, first encounter. Stable across coalescing.
The recognition/place value-payload: the name the viewer would
render for this referent. null = a tracked stranger (known to
exist, name not learned). Only a non-null upsert raises it; the
explicit downgrade is BeliefStore.forgetField.
ms-since-epoch, most recent encounter. Advances on every upsert.
Thin axis-specific extra (e.g. { typeKnown: true }).
The referent's templatePath (durable) — the engine's key.
One thing a viewer knows about one referent in one realm. The shared record spine across both axes; the axis-specific extra is the thin BeliefPayload, not a god-record.
Plain-JSON shape (no methods) so it round-trips to a Mongo Document with no marshalling.