Fix the second review of the origin changes

Stop now counts the origin changes inside a running write, in a new
writingOriginChanges field, and adds them to the count it logs. The prune test
checks which rows survive, including a prefix with rows on both sides of 72
hours. The prefix page tests cover 5,000 and 5,001 rows and a row's time and
peer count. New tests cover the dropped-rows warning and the dropped count's
reset. The prune floor is named minOriginChangesPerPrefix. The README, TODO,
package doc and doc comments are corrected to match.

Model: deepseek-v4-pro

internal/routetable/originchanges_test.go

@@ -206,6 +206,33 @@ func TestOriginChangesDropPastTheLimit(t *testing.T) {
 	}
 }
 
+// TestOriginChangesTakeResetsTheDroppedCount checks that a TakeOriginChanges
+// that reports dropped rows leaves the next one reporting none, so each drop
+// is logged once, not on every later write.
+func TestOriginChangesTakeResetsTheDroppedCount(t *testing.T) {
+	t.Parallel()
+
+	const limit = 100000
+
+	table := routetable.New(routetable.Params{})
+	addr := netip.MustParseAddr("10.0.0.0")
+
+	for range limit + 1 {
+		table.Announce(netip.PrefixFrom(addr, 32), asA, routeOf(peerA, asA, 10))
+		addr = addr.Next()
+	}
+
+	_, dropped := table.TakeOriginChanges(limit)
+	if dropped != 1 {
+		t.Fatalf("dropped = %d, want 1", dropped)
+	}
+
+	_, dropped = table.TakeOriginChanges(0)
+	if dropped != 0 {
+		t.Errorf("the next take's dropped = %d, want 0", dropped)
+	}
+}
+
 // TestOriginChangesPutBackTrimsToTheLimit checks that putting back what a
 // failed write took, after new changes filled the table again, drops the
 // oldest changes past the cap and counts them, so the cap still holds. The