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/channelserver/handlers_session.go

@@ -283,6 +283,11 @@ func logoutPlayer(s *Session) {
 		}
 	}
 
+	// Flush and close capture file before closing the connection.
+	if s.captureCleanup != nil {
+		s.captureCleanup()
+	}
+
 	// NOW do cleanup (after save is complete)
 	s.server.Lock()
 	delete(s.server.sessions, s.rawConn)