state: preserve previous primary when all HA advertisers unhealthy

electPrimaryRoutes' all-unhealthy fallback picked candidates[0]
(lowest NodeID) regardless of who was prev. Under cable-pull
semantics IsOnline lags reality (long-poll TCP half-open), so
both routers stay in candidates and both go Unhealthy via the
prober — the fallback then churned primary to a node that was
itself unreachable.

Prefer prev when still in candidates; fall through to
candidates[0] only when prev is gone. Anti-blackhole holds.

Update the property test reference model and split the unit
test into existence (KeepsAPrimary) and identity
(PreservesPrevious) cases.

Fixes #3203