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

config.example.json

@@ -31,6 +31,14 @@
     "RawEnabled": false,
     "OutputDir": "save-backups"
   },
+  "Capture": {
+    "Enabled": false,
+    "OutputDir": "captures",
+    "ExcludeOpcodes": [],
+    "CaptureSign": true,
+    "CaptureEntrance": true,
+    "CaptureChannel": true
+  },
   "DebugOptions": {
     "CleanDB": false,
     "MaxLauncherHR": false,