seedFromHistory

Seed subgraph hosts from checkpoint history (see namespace-from-history.collectSubgraphHostNamespaces) so subgraph cards render on thread refresh without waiting for the depth-1 SSE replay. Supplies the host/promotion decision from history instead of the live strict-prefix heuristic, then reuses the same #ensureShadow / #promoted / #commit path. Idempotent: never downgrades an entry that already reached a terminal state.