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

network/pcap/recording_conn.go

@@ -0,0 +1,65 @@
+package pcap
+
+import (
+	"encoding/binary"
+	"erupe-ce/network"
+	"sync"
+	"time"
+)
+
+// RecordingConn wraps a network.Conn and records all packets to a Writer.
+// It is safe for concurrent use from separate send/recv goroutines.
+type RecordingConn struct {
+	inner   network.Conn
+	writer  *Writer
+	startNs int64
+	mu      sync.Mutex
+}
+
+// NewRecordingConn wraps inner, recording all packets to w.
+// startNs is the session start time in nanoseconds (used as the time base).
+func NewRecordingConn(inner network.Conn, w *Writer, startNs int64) *RecordingConn {
+	return &RecordingConn{
+		inner:   inner,
+		writer:  w,
+		startNs: startNs,
+	}
+}
+
+// ReadPacket reads from the inner connection and records the packet as client-to-server.
+func (rc *RecordingConn) ReadPacket() ([]byte, error) {
+	data, err := rc.inner.ReadPacket()
+	if err != nil {
+		return data, err
+	}
+	rc.record(DirClientToServer, data)
+	return data, nil
+}
+
+// SendPacket sends via the inner connection and records the packet as server-to-client.
+func (rc *RecordingConn) SendPacket(data []byte) error {
+	err := rc.inner.SendPacket(data)
+	if err != nil {
+		return err
+	}
+	rc.record(DirServerToClient, data)
+	return nil
+}
+
+func (rc *RecordingConn) record(dir Direction, data []byte) {
+	var opcode uint16
+	if len(data) >= 2 {
+		opcode = binary.BigEndian.Uint16(data[:2])
+	}
+
+	rec := PacketRecord{
+		TimestampNs: time.Now().UnixNano(),
+		Direction:   dir,
+		Opcode:      opcode,
+		Payload:     data,
+	}
+
+	rc.mu.Lock()
+	_ = rc.writer.WritePacket(rec)
+	rc.mu.Unlock()
+}