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/entrance_server.go

@@ -108,7 +108,10 @@ func (s *Server) handleEntranceServerConnection(conn net.Conn) {
 	}
 
 	// Create a new encrypted connection handler and read a packet from it.
-	cc := network.NewCryptConn(conn, s.erupeConfig.RealClientMode, s.logger)
+	var cc network.Conn = network.NewCryptConn(conn, s.erupeConfig.RealClientMode, s.logger)
+	cc, captureCleanup := startEntranceCapture(s, cc, conn.RemoteAddr())
+	defer captureCleanup()
+
 	pkt, err := cc.ReadPacket()
 	if err != nil {
 		s.logger.Warn("Error reading packet", zap.Error(err))