The statelessPeers map permanently blacklists peers that return empty
responses for the entire Sync() cycle. In partial state mode, the faster
account advancement (due to skipping storage/code for non-tracked
contracts) creates bursty request patterns that can trigger transient
empty responses. Combined with the permanent blacklist, this causes a
cascade where all peers get banned and sync stalls permanently.
Replace the permanent map[string]struct{} with map[string]time.Time to
track when each peer was marked. For partial state mode, peers are given
a 30-second cooldown instead of permanent banishment. After the cooldown
expires, the peer is eligible for task assignment again. Full sync mode
behavior is unchanged (permanent blacklist preserved).
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