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

cmd/replay/compare.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"fmt"
+
+	"erupe-ce/network"
+	"erupe-ce/network/pcap"
+)
+
+// PacketDiff describes a difference between an expected and actual packet.
+type PacketDiff struct {
+	Index          int
+	Expected       pcap.PacketRecord
+	Actual         *pcap.PacketRecord // nil if no response received
+	OpcodeMismatch bool
+	SizeDelta      int
+}
+
+func (d PacketDiff) String() string {
+	if d.Actual == nil {
+		return fmt.Sprintf("#%d: expected 0x%04X (%s), got no response",
+			d.Index, d.Expected.Opcode, network.PacketID(d.Expected.Opcode))
+	}
+	if d.OpcodeMismatch {
+		return fmt.Sprintf("#%d: opcode mismatch: expected 0x%04X (%s), got 0x%04X (%s)",
+			d.Index,
+			d.Expected.Opcode, network.PacketID(d.Expected.Opcode),
+			d.Actual.Opcode, network.PacketID(d.Actual.Opcode))
+	}
+	return fmt.Sprintf("#%d: 0x%04X (%s) size delta %+d bytes",
+		d.Index, d.Expected.Opcode, network.PacketID(d.Expected.Opcode), d.SizeDelta)
+}
+
+// ComparePackets compares expected server responses against actual responses.
+// Only compares S→C packets (server responses).
+func ComparePackets(expected, actual []pcap.PacketRecord) []PacketDiff {
+	expectedS2C := pcap.FilterByDirection(expected, pcap.DirServerToClient)
+	actualS2C := pcap.FilterByDirection(actual, pcap.DirServerToClient)
+
+	var diffs []PacketDiff
+	for i, exp := range expectedS2C {
+		if i >= len(actualS2C) {
+			diffs = append(diffs, PacketDiff{
+				Index:    i,
+				Expected: exp,
+				Actual:   nil,
+			})
+			continue
+		}
+		act := actualS2C[i]
+		if exp.Opcode != act.Opcode {
+			diffs = append(diffs, PacketDiff{
+				Index:          i,
+				Expected:       exp,
+				Actual:         &act,
+				OpcodeMismatch: true,
+			})
+		} else if len(exp.Payload) != len(act.Payload) {
+			diffs = append(diffs, PacketDiff{
+				Index:     i,
+				Expected:  exp,
+				Actual:    &act,
+				SizeDelta: len(act.Payload) - len(exp.Payload),
+			})
+		}
+	}
+
+	// Extra actual packets beyond expected.
+	for i := len(expectedS2C); i < len(actualS2C); i++ {
+		act := actualS2C[i]
+		diffs = append(diffs, PacketDiff{
+			Index:    i,
+			Expected: pcap.PacketRecord{},
+			Actual:   &act,
+		})
+	}
+
+	return diffs
+}