`RoundRef.swap` parked the outgoing round in a `previous` field that no
production code read, keeping a second full CPU copy of the model
resident for an extra round.

Addresses the OOM kills on the foundation validator (8 since 2026-08-22,
none in the five weeks before), each of which finalized the open round
with whatever it had — round 8907858 ended with roster 220, scored 0.

Removes one of the two resident copies: expect the interval between
kills to lengthen rather than the kills to stop.