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/routewatch/prefixhandler_originchanges_internal_test.go

@@ -1,12 +1,16 @@
 package routewatch
 
 import (
+	"bytes"
+	"encoding/json"
 	"errors"
+	"log/slog"
 	"net/netip"
 	"slices"
 	"testing"
 
 	"git.eeqj.de/sneak/routewatch/internal/database"
+	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/routetable"
 )
 
@@ -125,3 +129,53 @@ func TestPrefixHandlerWritesManyOriginChangesOverSeveralFlushes(t *testing.T) {
 		t.Errorf("WriteOriginChanges held %v changes, want %v", store.counts, want)
 	}
 }
+
+// TestPrefixHandlerLogsTheDroppedOriginChanges fills the routing table with
+// one origin change more than it keeps, then flushes. The warning the flush
+// logs must carry the dropped count.
+func TestPrefixHandlerLogsTheDroppedOriginChanges(t *testing.T) {
+	t.Parallel()
+
+	var logs bytes.Buffer
+
+	db := newTestDatabase(t)
+	h := prefixHandlerWithoutTimer(db)
+	h.logger = &logger.Logger{
+		Logger: slog.New(slog.NewJSONHandler(&logs,
+			&slog.HandlerOptions{Level: slog.LevelWarn})),
+	}
+
+	announceManyPrefixes(h.table, 100001)
+
+	h.flushBatch()
+
+	var warning struct {
+		Msg     string `json:"msg"`
+		Dropped *int   `json:"dropped"`
+	}
+
+	found := false
+
+	for line := range bytes.SplitSeq(bytes.TrimSpace(logs.Bytes()), []byte("\n")) {
+		warning.Dropped = nil
+
+		err := json.Unmarshal(line, &warning)
+		if err != nil {
+			t.Fatalf("want a JSON log line, got %q: %v", line, err)
+		}
+
+		if warning.Msg != "Origin changes dropped" {
+			continue
+		}
+
+		found = true
+
+		if warning.Dropped == nil || *warning.Dropped != 1 {
+			t.Errorf("the warning's dropped = %v, want 1", warning.Dropped)
+		}
+	}
+
+	if !found {
+		t.Errorf("no warning logged the dropped origin changes")
+	}
+}