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
