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/signserver/sign_server.go

@@ -104,13 +104,21 @@ func (s *Server) handleConnection(conn net.Conn) {
 	}
 
 	// Create a new session.
+	var cc network.Conn = network.NewCryptConn(conn, s.erupeConfig.RealClientMode, s.logger)
+	cc, captureCleanup := startSignCapture(s, cc, conn.RemoteAddr())
+
 	session := &Session{
-		logger:    s.logger,
-		server:    s,
-		rawConn:   conn,
-		cryptConn: network.NewCryptConn(conn, s.erupeConfig.RealClientMode, s.logger),
+		logger:         s.logger,
+		server:         s,
+		rawConn:        conn,
+		cryptConn:      cc,
+		captureCleanup: captureCleanup,
 	}
 
 	// Do the session's work.
 	session.work()
+
+	if session.captureCleanup != nil {
+		session.captureCleanup()
+	}
 }