Adds a third protection category to the dropper, scoring peers by
per-peer tx-request response latency. Fast peers are harder to drop;
peers that chronically time out (their EMA drifts toward the 5s
timeout sample) score low and are normal drop candidates.
PeerInclusionStats gains RequestLatencyEMA (time.Duration) and
RequestSamples (int64). The stats adapter in backend.go copies them
from txtracker.PeerStats. The scoring function returns 1/EMA once
the peer has >= MinLatencySamples (10) recorded samples — an
under-sampled peer scores 0 and is filtered by the existing
"score <= 0" rule, preventing a single lucky-fast reply from
displacing established peers.
Adds three unit tests via protectedPeersByPool for the basic
top-N selection, the bootstrap guard, and per-pool independence.
c82be6827f