eth/txtracker: prune peer stats on disconnect

Peer stats were never pruned, so the peers map grew with every peer
ever seen. The EMA decay loop and stats copy iterated all historical
peers on every block/query.

Add NotifyPeerDrop(peer) that deletes the peer's stats entry. Called
from handler.unregisterPeer alongside txFetcher.Drop.

eth/handler.go

@@ -406,6 +406,7 @@ func (h *handler) unregisterPeer(id string) {
 	}
 	h.downloader.UnregisterPeer(id)
 	h.txFetcher.Drop(id)
+	h.txTracker.NotifyPeerDrop(id)
 
 	if err := h.peers.unregisterPeer(id); err != nil {
 		logger.Error("Ethereum peer removal failed", "err", err)

eth/txtracker/tracker.go

@@ -85,6 +85,14 @@ func (t *Tracker) Start(chain Chain) {
 	go t.loop()
 }
 
+// NotifyPeerDrop removes a disconnected peer's stats to prevent unbounded
+// growth. Safe to call from any goroutine.
+func (t *Tracker) NotifyPeerDrop(peer string) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	delete(t.peers, peer)
+}
+
 // Stop shuts down the tracker.
 func (t *Tracker) Stop() {
 	t.sub.Unsubscribe()