feat(network): add protocol packet capture and replay system

Add a recording and replay foundation for the MHF network protocol.
A RecordingConn decorator wraps network.Conn to transparently capture
all decrypted packets to binary .mhfr files, with zero handler changes
and zero overhead when disabled.

- network/pcap: binary capture format (writer, reader, filters)
- RecordingConn: thread-safe Conn decorator with direction tracking
- CaptureOptions in config (disabled by default)
- Capture wired into all three server types (sign, entrance, channel)
- cmd/replay: CLI tool with dump, json, stats, and compare modes
- 19 new tests, all passing with -race

server/entranceserver/sys_capture.go

@@ -0,0 +1,92 @@
+package entranceserver
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"path/filepath"
+	"time"
+
+	"erupe-ce/network"
+	"erupe-ce/network/pcap"
+
+	"go.uber.org/zap"
+)
+
+// startEntranceCapture wraps a Conn with a RecordingConn if capture is enabled for entrance server.
+func startEntranceCapture(s *Server, conn network.Conn, remoteAddr net.Addr) (network.Conn, func()) {
+	capCfg := s.erupeConfig.Capture
+	if !capCfg.Enabled || !capCfg.CaptureEntrance {
+		return conn, func() {}
+	}
+
+	outputDir := capCfg.OutputDir
+	if outputDir == "" {
+		outputDir = "captures"
+	}
+	if err := os.MkdirAll(outputDir, 0o755); err != nil {
+		s.logger.Warn("Failed to create capture directory", zap.Error(err))
+		return conn, func() {}
+	}
+
+	now := time.Now()
+	filename := fmt.Sprintf("entrance_%s_%s.mhfr",
+		now.Format("20060102_150405"),
+		sanitizeAddr(remoteAddr.String()),
+	)
+	path := filepath.Join(outputDir, filename)
+
+	f, err := os.Create(path)
+	if err != nil {
+		s.logger.Warn("Failed to create capture file", zap.Error(err), zap.String("path", path))
+		return conn, func() {}
+	}
+
+	startNs := now.UnixNano()
+	hdr := pcap.FileHeader{
+		Version:        pcap.FormatVersion,
+		ServerType:     pcap.ServerTypeEntrance,
+		ClientMode:     byte(s.erupeConfig.RealClientMode),
+		SessionStartNs: startNs,
+	}
+	meta := pcap.SessionMetadata{
+		Host:       s.erupeConfig.Host,
+		Port:       int(s.erupeConfig.Entrance.Port),
+		RemoteAddr: remoteAddr.String(),
+	}
+
+	w, err := pcap.NewWriter(f, hdr, meta)
+	if err != nil {
+		s.logger.Warn("Failed to initialize capture writer", zap.Error(err))
+		_ = f.Close()
+		return conn, func() {}
+	}
+
+	s.logger.Info("Capture started", zap.String("file", path))
+
+	rc := pcap.NewRecordingConn(conn, w, startNs)
+	cleanup := func() {
+		if err := w.Flush(); err != nil {
+			s.logger.Warn("Failed to flush capture", zap.Error(err))
+		}
+		if err := f.Close(); err != nil {
+			s.logger.Warn("Failed to close capture file", zap.Error(err))
+		}
+		s.logger.Info("Capture saved", zap.String("file", path))
+	}
+
+	return rc, cleanup
+}
+
+func sanitizeAddr(addr string) string {
+	out := make([]byte, 0, len(addr))
+	for i := 0; i < len(addr); i++ {
+		c := addr[i]
+		if c == ':' {
+			out = append(out, '_')
+		} else {
+			out = append(out, c)
+		}
+	}
+	return string(out)
+}